Hypoxic pulmonary vasoconstriction (HPV) may be mediated, in part, by an oxygen-sensing mechanism intrinsic to pulmonary arterial smooth muscle. It has been proposed that hypoxia inhibits a K+ conductance, which promotes membrane depolarization, subsequent activation of L-type Ca2+ channels and ultimately constriction. We have monitored hypoxia-induced changes in the intracellular Ca2+ concentration ([Ca2+]i) of single myocytes isolated from the rat pulmonary arterial tree using microspectrofluorimetry and ratiometric measurement of indo-1 fluorescence. The basal level of [Ca2+]i was in the range 35-80 nM and cells were quiescent at rest exhibiting no spontaneous oscillations in [Ca2+]i. When the extracellular K+ concentration ([K+]o) was raised to 20 mM, the [Ca2+]i increased from [similar]60 to [similar]100 nM. This increase was abolished by nifedipine, demonstrating the presence, and need for activation, of functional voltage-gated L-type Ca2+ channels. Hypoxia (PO2 [less than or equal to] 30 mmHg; throughout) had little effect on the resting [Ca2+]i in myocytes isolated from either the main intrapulmonary artery, or its primary, secondary or tertiary branches. However, upon raising the [Ca2+]i by increasing [K+]o to 20 mM, hypoxia was found to lower [Ca2+]i, from [similar]110 to [similar]70 nM, in cells isolated from the main conduit and primary branches of the intrapulmonary artery. In marked contrast, when [Ca2+]i was raised, by increasing [K+]o to 20 mM, in myocytes isolated from secondary and tertiary branches of the intrapulmonary artery, hypoxia induced a further reversible increase in the [Ca2+]i from [similar]160 to [similar]240 nM. Neither hypoxia alone nor in combination with 20 mM K+o induced any increase in the [Ca2+]i in the presence of nifedipine. We conclude that hypoxia may modulate [Ca2+]i in rat pulmonary artery myocytes only following its elevation by a depolarizing stimulus.