The effect of hypoxia on mechanical and electrical properties of smooth muscle from the rat portal vein.

The effect of hypoxia on the electrical and mechanical activity of rat portal vein smooth muscle were investigated using intracellular microelectrode technique in combination with contraction force measurements. In control conditions the "resting" potential of the muscle cells was -58.0 +/- 0.1 mV (mean +/- S.E.). Bursts of action potentials, 5-10s long, appeared at regular intervals (2-3/min) in association with phasic contractions. In hypoxia (PO2 at about 10 mm Hg) there was a marked decrease in force and, often, a moderate increase in rate of the spontaneous contractions. Electrically, these changes corresponded to a decrease in length and an increase in frequency of the bursts of action potentials. The general level of membrane polarization and individual action potentials were not affected. In prolonged hypoxia there was a tendency towards dissociation of the electrical and mechanical activities. Increasing [K+]0 or reducing [Na+]0 produced a restoration of the hypoxically depressed force development. Qualitatively similar results with respect to the depression of myogenic activity by hypoxia, and its alleviation in decreased [K+]0 and reduced [Na+]0, were obtained in low Ca (1 mM) solutions. The effects of hypoxia, and their variation with [K+]0 and [Na+]0, could be explained on the basis of pH dependent electro-mechanical uncoupling.