Smooth muscle intracellular pH: measurement, regulation, and function.

Smooth muscle performs many functions that are essential for the normal working of the human body. Changes in pH are thought to affect many aspects of smooth muscle. Despite this, until recently little was known about either intracellular pH (pHi) values or pHi regulation in smooth muscle. Recent work measuring pHi with either microelectrodes or nuclear magnetic resonance spectroscopy is now providing some of this much needed information for smooth muscles. From these studies, it can be concluded tentatively that pHi is the same in different smooth muscles, approximately 7.06 (37 degrees C). This value is very close to those obtained in cardiac and skeletal muscle. It is clear that H+ is not in equilibrium across the smooth muscle membrane; i.e., pHi is regulated. Preliminary results in smooth muscle suggest that certain aspects of this regulation are different from that described for other muscle types. Changes in pHi have been found to produce marked effects on contraction in smooth muscle. Of particular interest is the fact that, unlike striated muscles, some smooth muscles can product more force during an intracellular acidification.