Introduction

Smooth muscle is found within the walls of hollow organs and tubes of the body such as the uterus, stomach, ureter and blood vessels. Its main function is to move liquids or objects through these tubes, which it does by altering its contractile state. Its correct functioning is therefore vital to many activities, for example breathing, reproductive ability, voiding urine and controlling blood pressure. The control of the contraction of smooth muscle is accomplished by neuronal, hormonal and local mechanisms. This chapter focuses on one such local mediator – pH – and considers how pH may be altered by normal and pathological processes. Given its potent ability to modify smooth muscle contractile activity, it is clearly necessary that pH is regulated. Therefore, what is known about this process in smooth muscle is discussed, and how changes in pH affect contractility is described.

Techniques used to measure pH in smooth muscles

Measurement of extracellular pH (pHe) in smooth muscles is generally straightforward. In single cell studies or perfused organs pHe will be that of the perfusate. This is usually 7.4, to match that of plasma. For measurement of surface pH, microelectrodes can be placed against the cell or tissue (Thomas, 1984).

The measurement of intracellular pH (pHi) in smooth muscle has now become routine, mainly because of the development of pH-sensitive fluorescent indicators. The small size of smooth muscle cells coupled with their contractile nature had largely prevented the successful use of pH-sensitive microelectrodes (Aickin, 1984).