Knowledge of pleural liquid pressure (Pliq) and composition is crucial for studies concerning intrapleural fluid dynamics, and pleural fluid turnover. We measured Pliq at intercostal and costal levels in anaesthetized spontaneously breathing rabbits using a minimally invasive method that assures a long-lasting hydraulic continuity between the pleural liquid and the recording system. Polyethylene tubes were glued either to the exposed endothoracic fascia or inserted into a rib to provide a sealed connection to the recording system. After inducing a pneumothorax with nitrous oxide (N2O) via an intrapleural cannula, a hole ([similar]0·7 mm2) was pierced in the parietal pleura through the tube lumen. The tubes were then connected to pressure transducers and the whole system was filled with heparinized saline to the level of the parietal pleura; finally the pneumothorax was removed after N2O washout and Pliq recordings were performed. A different kind of tube was used to obtain microsamples of pleural fluid (2·5-3 µl) during spontaneous breathing; colloid osmotic pressure of the microsamples ([Pi]liq) was measured with an osmometer, and averaged 9·3 ± 1·5 cmH2O (n = 70 samples). When pooled and plotted against lung height end-expiratory intercostal and costal Pliq data scattered along a single regression line with a slope of -0·83 and -0·90 cmH2O cm-1 in supine and prone animals, respectively. End-inspiratory costal Pliq was significantly more subatmospheric than intercostal in the ventral region of the chest (P < 0·05), and less subatmospheric in the dorsal region, regardless of posture. The techniques presented here could be helpful in gaining a greater insight into the physiology and pathophysiology of the pleural space in terms of pleural fluid dynamics and turnover.

The extrusion mechanism for intracellular Mg2+ was investigated in voltage-clamped snail neurones using Mg2+-sensitive microelectrodes and mag-fura-2. The intracellular free magnesium ion concentration ([Mg2+]i) of snail neurones voltage clamped to -60 mV was estimated to be 0·57 ± 0·06 mM (mean ± S.E.M.; n = 12) using Mg2+-sensitive microelectrodes and 0·62 ± 0·05 mM (n = 15) using mag-fura-2 . Raising extracellular MgCl2 from 5 to 20 mM caused an average increase in [Mg2+]i of 0·25 ± 0·04 mM (n = 7). In three experiments, removing extracellular MgCl2 caused an average decrease in [Mg2+]i of 0·1 mM. Replacing extracellular Na+ with N-methyl-D-glucamine (NMDG) caused a rise in [Mg2+]i of 1·8 ± 0·5 mM (n = 7); [Mg2+]i recovered to resting levels when extracellular Na+ was restored. Iontophoretic injections of MgCl2 were used to raise [Mg2+]i. The rate of recovery from such increases in [Mg2+]i (calculated from the slope of the recovery) was inhibited by 85-100 % (n = 5) in the absence of extracellular Na2+ compared with control conditions. Raising extracellular Ca2+ from 7 to 35 mM caused a reversible rise in [Mg2+]i of 0·4 ± 0·05 mM (mean ± S.E.M., n = 7). It was concluded that in snail neurones the main mechanism for [Mg2+]i extrusion is a Na+-Mg2+ exchanger which may be partially inhibited be high extracellular Ca2+ concentrations.

The experiments were designed to elucidate the mechanism of thiopentone-induced inhibition of contractile responses in isolated human epigastric arteries. Segments of human epigastric arteries were obtained from patients who underwent elective or emergency caesarean section, placed in standard physiological salt solution (PSS), cut into rings at 3 mm intervals and suspended in organ baths for recording of isometric contractions at 37¡C, and pH 7·4. Three protocols were employed to examine the inhibitory effect of thiopentone: (a) concentration-dependent effect on 10-7 M noradrenaline (NA)- or high-K+ (40 mM)-induced contractions; (b) effect on NA-induced extra- and intracellular Ca2+-dependent contractions and (c) effect on the dose-response curve when Ca2+ is restored to Ca2+-depleted rings in Ca2+-free 40 mM K+-depolarizing medium. Thiopentone (1 × 10-6-4 × 10-3 M) caused concentration-dependent relaxation of both NA- and high-K+-induced contractions. The magnitudes of both precontractions were not significantly different but the IC50 values for thiopentone relaxation of high-K+ contractions were significantly lower than for NA contractions. Thiopentone (10-4 M) significantly attenuated the phasic (intracellular Ca2+ dependent) contractile responses to 10-5 M NA in Ca2+-free PSS as well as the tonic (extracellular Ca2+ dependent) contractions upon restoration of Ca2+. In contrast, nifedipine (1 µM) did not modify the phasic response but significantly attenuated the tonic response. Thiopentone (10-4 M) also almost completely abolished concentration-dependent Ca2+-induced contractions in K+-depolarized Ca2+-depleted rings. The results suggest that in the smooth muscle of human epigastric arteries, thiopentone-induced relaxation is non-specific and is associated with impairment of Ca2+ supply from both extracellular and intracellular pools.

This study focuses on the potential role for nitric oxide on the actions of the parasympathetic innervation to the heart. Earlier, we showed that the nitric oxide synthase (NOS) inhibitor N[omega]- nitro-L-arginine methyl ester (L-NAME) reduced the bradycardia induced by stimulation of vagal efferent motor fibres and that these effects are reversible through administration of the NOS substrate L-arginine. In the present study, we show that D-arginine does not reverse the effects of the inhibitors and confirm that they are reversed by L-arginine. Another NOS inhibitor, NG -nitro-L-arginine (L-NOARG), produced similar effects which were not reversed by L-arginine. In an examination of the effect of increasing NO availability with the NO donor sodium nitroprusside the vagally induced bradycardia was enhanced at all frequencies tested. In a separate series, the effects of NOS inhibitors and NO donors on the dromotropic actions of the vagus were examined. The NOS inhibitor L-NAME, reduced the increase in atrio-ventricular conduction delay normally induced by efferent vagal stimulation at all frequencies tested both in the 'paced' and 'unpaced' heart. Further, sodium nitroprusside enhanced this delay. Overall the study indicates that NO has an important facilitatory role on both the chronotropic and dromotropic actions of the vagus nerve on the heart and that NO may be a rate-limiting factor in the cardiac responses to vagal stimulation.

After 2 months of streptozotocin-induced diabetes in rats, the membrane potential of the diaphragm muscle when measured in vitro at 30¡C was unchanged but the tetanic tension, the half-relaxation time of the isometric twitch and the fatigue resistance were each reduced. Treatment of the diabetic rats with the antihyperglycaemic agent metformin prevented the decrease in half-relaxation time and the greater degree of fatigue in the diaphragms. The possibility that changes in H+ and cyclic AMP concentrations in the diabetic muscles contributed to the decreased contractile function and that metformin acted by attenuating these changes is discussed.

We investigated the effects of glucose 6-phosphate (G6P) on skeletal muscle contractile apparatus and sarcoplasmic reticulum (SR) function. Using rat extensor digitorum longus fibres, the presence of 5 mM G6P decreased the Ca2+ sensitivity of both force production and actomyosin ATPase (AM-ATPase) activity. Conversely, maximal Ca2+-activated force was unaffected while maximal AM-ATPase activity was increased by 37 %. In SR vesicles isolated from rat gastrocnemius, G6P markedly altered Ca2+ handling. It increased Ca2+-stimulated Ca2+-ATPase activity but depressed the net rate of Ca2+ uptake. This latter effect appears to be due to G6P-stimulated Ca2+ release. When G6P was added to Ca2+-loaded vesicles, a small, transient release of Ca2+ was elicited. In addition, G6P lowered the threshold for Ca2+-induced Ca2+ release but depressed the net rates of both AgNO3- and caffeine-induced releases. It is possible that the accumulation of G6P during muscular activity may adversely affect muscle force production and contribute to the fatigue process via its action on the contractile apparatus and SR.

The present study was carried out to determine whether the increased salt intake induced by increased specific sodium appetite in pregnant rats modifies water-salt homeostasis throughout pregnancy. Two groups of pregnant rats were used, one fed ad libitum with a normal sodium (NS) diet consisting of standard rat chow and distilled water, and the other fed with a high-sodium (HS) diet with free access to chow, distilled water plus saline solution (1·5 % NaCl). Virgin rats in dioestrus were also studied as non-pregnant controls. Pregnant animals were studied on days 4, 9, 14, 20 and 21 of gestation at which time body weight, water and saline intake, sodium excretion, plasma atrial natriuretic peptide (ANP) and arginine vasopressin (AVP) concentrations, as well as plasma osmolality were determined. Data showed that water intake was higher in the NS group, but total fluid intake (water plus saline) was higher in the HS group throughout pregnancy. Dietary sodium intake was the same for both groups but total sodium intake (chow plus saline) was 60-98 % higher in the HS rats. Pregnant HS rats excreted more fluid (35-50 %) and sodium (up to 100 %) compared with NS rats, indicating that the animals could change their renal excretion in response to a 2·5-fold higher dietary sodium intake compared with the control level. Salt satiety during pregnancy did not modify plasma ANP concentration. In both groups of pregnant rats ANP levels increased 3-fold on day 14 without significant alteration in sodium excretion, suggesting that the natriuretic action of ANP is attenuated at least after the second week of pregnancy. High sodium intake did not change plasma AVP concentration or osmolality and both groups showed the same gradual decrease in plasma osmolality ([similar]8 mosmol kg-1) at the end of pregnancy that was not accompanied by decreased plasma AVP concentration. The present data show that rats maintain the special homeostatic equilibrium that occurs in normal pregnancy even when they are allowed to increase sodium intake to satisfy their salt appetite during this period of the reproductive cycle.

The abdominal circulation contains a high proportion of the total blood volume and this can change either passively in response to changes in vascular distending pressure or actively (termed a capacitance response) to changes in sympathetic nervous activity. The liver is the largest abdominal organ and this study was designed to evaluate its potential contribution to overall vascular capacitance and compliance. In chloralose anaesthetized dogs, the liver was vascularly isolated, perfused through the portal vein and hepatic artery at either constant pressures or constant flows and drained from the hepatic veins at constant pressure. Changes in vascular resistance were assessed from changes in inflow pressures or flows and hepatic blood volume was determined by differences between net inflow and outflow. During constant flow perfusion the change in hepatic volume (capacitance change) in response to supramaximal stimulation of sympathetic nerves at 16 Hz was (mean ± S.E.M.) -2·40 ± 0·61 ml (kg body weight)-1. This response was not significantly different during constant pressure perfusion. The changes in portal venous and hepatic arterial pressures during stimulation at constant flow perfusion were +0·67 ± 0·13 and +4·92 ± 0·67 kPa, respectively. The compliance of the liver, assessed as the change in volume to a change in hepatic venous pressure, was +5·44 ± 0·18 ml kg-1 kPa-1. These results indicate that the liver has a major capacitance role, comparable to that of the canine spleen and, in addition, is highly compliant. No evidence was found to suggest that a sphincter on the hepatic outflow exists. Assuming similar responses occur in humans, who do not possess a large contractile spleen, the liver would be the most important controllable blood reservoir in the body.

Investigations of intestinal secretion are often performed under anaesthesia. This study evaluates the influence of anaesthetic agents on the intestinal secretion induced by cholera toxin (CT) in the pig. CT was instilled for 4 h in ligated jejunal loops under anaesthesia with halothane, saffan, [alpha]-chloralose, or propofol. Cardiovascular parameters, blood gas data, plasma cortisol levels, net fluid accumulation, intraluminal mediators (serotonin (5-HT), prostaglandin E2 (PGE2)) and electrolyte concentrations in the accumulated fluid were determined. The systolic blood pressure and heart rate was highest for saffan-anaesthetized pigs (blood pressure: saffan > [alpha]-chloralose > propofol = halothane; heart rate: saffan > [alpha]-chloralose = propofol = halothane), while blood gases and cortisol levels were within the same range. CT induced a dose-dependent fluid accumulation under all four anaesthetics. The fluid accumulation was significantly higher in pigs treated with saffan, [alpha]-chloralose and propofol than in halothane-treated pigs (saffan = [alpha]-chloralose > propofol > halothane). There was no significant difference in electrolyte concentrations in the accumulated fluid or in the luminal content of 5-HT and PGE2 between anaesthetics. The results demonstrate that anaesthetic agents profoundly influence the secretory response in the small intestine and indicate the importance of the choice of anaesthetic in this type of experiment.

Parotid secretion has been investigated in anaesthetized lambs in the presence and absence of N[omega]- nitro-L-arginine methyl ester (L-NAME) and sodium nitroprusside to block de novo synthesis of nitric oxide (NO). Following administration of L-NAME the basal rate of flow was unaffected and changes in electrolyte secretion failed to achieve statistical significance but there was a significant fall in the basal rate of protein secretion. The flow of parotid saliva which occurred in response to stimulation of the parasympathetic innervation was reduced by 34 % and sodium output was reduced in approximately the same proportion. L-NAME had no significant effect on these parameters during stimulation of the sympathetic innervation. During combined stimulation of the parasympathetic and sympathetic innervations L-NAME caused a reduction in parotid salivary flow and sodium output which was roughly the same as that observed during parasympathetic stimulation alone. However, L-NAME caused a much greater reduction in protein output during each of these experimental protocols: -92 % during parasympathetic stimulation, -63 % during sympathetic stimulation, and -60 % during combined stimulation. Whereas the absolute amount of protein secreted was reduced after L-NAME in each instance, the extent of potentiation of protein output recorded during combined stimulation was increased roughly fivefold. It is concluded that the output of protein in response to autonomic stimulation exhibits a greater NO dependence than either the flow of saliva or secretion of electrolytes in this gland.

The autonomic control of submandibular secretion has been investigated in fully weaned, anaesthetized calves 7 weeks after birth. Stimulation of the parasympathetic (chorda-lingual) innervation invariably produced a flow of saliva, the rate of which was frequency dependent over the range 2-8 Hz continuously. Neither the rate of flow nor the output of protein was enhanced by stimulating in bursts at relatively high frequencies. Stimulation of the sympathetic innervation (20 Hz for 1 s at 10 s intervals) alone produced a much slower flow of saliva but with a considerably higher protein content. Stimulation of both together produced no greater flow of saliva than occurred with either alone at the lower frequencies (2 and 4 Hz) but there was a pronounced synergy in respect of the secretion of protein. Following pre-treatment with propranolol (1·0 mg kg-1 I.V.), during on-going chorda-lingual stimulation at 4 Hz, intra-arterial injections of 1 nmol of either vasoactive intestinal peptide (VIP) or pituitary adenylate cyclase activating peptide (PACAP) elicited an increase in the flow and protein output of about the same order of magnitude. Calcitonin gene-related peptide (CGRP) also produced these same effects with roughly half the efficacy of VIP and PACAP but substance P had no detectable effect. It is concluded that VIP, PACAP and possibly CGRP are candidates for neurotransmitters with a role in the control of secretion in this gland.

For moderate work rates (i.e. below the lactate threshold, [theta]L), oxygen uptake ([Vdot]O2) approaches the steady state mono-exponentially. At higher work rates, the [Vdot]O2 kinetics are more complex, reflecting the delayed superimposition of an additional, slow component. The mechanisms of this ‘slow’ component are poorly understood. It has been demonstrated, however, that while a prior bout of supra-[theta]L cycling (with a 6 min recovery) does not significantly affect the [Vdot]O2 time course for a subsequent sub-[theta]L bout, it significantly speeds the [Vdot]O2 response to a subsequent supra-[theta]L bout (Gausche, Harmon, Lamarra & Whipp, 1989; Gerbino, Ward & Whipp, 1996). These investigators proposed that this speeding was a result of improved muscle perfusion during the exercise transient, possibly related to the residual metabolic acidaemia still present at the start of the subsequent exercise bout. To determine whether speeding of the [Vdot]O2 kinetics could also be induced by a bout of prior high-intensity exercise performed at a remote site (e.g. the arms), subjects each performed two 6 min bouts of high-intensity cycling (leg exercise: LE) at a work rate equivalent to 50 % of ‘[Delta]LE’ (the difference between maximum [Vdot]O2,LE and [theta]L,LE). On one occasion this was preceded by a 6 min period of cycling at 50 % [Delta]LE and, on another, by a similar period of arm-crank exercise (arm exercise: AE) at 50 % [Delta]AE; in each case, the work bouts were separated by 6 min of unloaded pedalling. Pulmonary gas exchange variables were derived breath-by-breath. During unloaded pedalling and at minute 6 of each work bout, arterialized venous blood samples were drawn from the dorsum of the heated hand or at the wrist for analysis of pH, lactate, pyruvate, noradrenaline (NAdr), adrenaline (Adr), and potassium (K+). The difference in [Vdot]O2 between minute 6 and 3 of each work bout ([Delta][Vdot]O2,[6-3]) and the 'partial' O2 deficit (O2 Def) provided indices of the slow phase of [Vdot]O2 kinetics. The initial AE and LE bouts resulted in similar degrees of metabolic (lactic) acidaemia; the residual acidaemia at the end of the subsequent 6 min recovery phase was also similar for the two protocols, as were [K+], [Adr] and [NAdr]. The subsequent LE bouts were associated with reductions in both [Delta][Vdot]O2,[6-3] and O2 Def, relative to control, with the effect being more marked when the work was preceded by a prior LE bout than a prior AE bout: [Delta][Vdot]O2,[6-3] averaging 32 and 56 % of control, respectively, and O2 Def 71 and 81 %. Consequently, the increase in [lactate] and decrease in pH induced in this second LE bout were smaller when preceded by prior leg exercise than prior arm exercise. It is therefore concluded that while metabolic acidaemia induced at a site remote from the legs is associated with a less prominent slow phase of the [Vdot]O2 kinetics for high-intensity leg exercise, a component specific to the involved contractile units appears to exert the dominant effect. The mechanisms underlying this response are, however, presently uncertain.

I am still puzzling over such questions as to what is the largest object caught in a spider's web and what are the limitations to building larger webs? Or, what might be the selective forces that have operated on the evolution of aglomerular kidneys that has occurred in a few marine teleost species? These are just a couple of the thought-provoking questions that are posed liberally throughout the latest (4th) edition of Eckert's Animal Physiology that has been revised by Randall, Burggren and French. For those students and teachers who found the earlier editions of this book instructional and a valuable resource, its extensive revision and expansion will be received with enthusiasm. The authors have produced an excellent textbook of animal physiology. The animal kingdom is so diverse, one wonders how such a book can be confined to 700 pages. The aim of the authors has been to carefully elucidate the general physiological principles and physico-chemical laws which govern all forms of animal life and then to illustrate these laws with examples of adaptations drawn from a wide range of organisms. This aim has been largely fulfilled and the authors are to be congratulated on their revision.