General features of electrical and mechanical properties of smooth muscle cells in the guinea-pig abdominal aorta.

The membrane potential in smooth muscle cells of the guinea-pig's abdominal aorta has a mean value of -57.2 mV. These cells are electrically connected and the space and time constant are 0.66 mm and 180 ms respectively. An increased K-concentration elicited a contraction at 20 mM and the maximum was reached at 77 mM. The maximum depolarization produced by a tenfold increase of [K]0 was 45 mV. Tetraethylammonium at concentrations exceeding 2 mM, depolarized the membrane, increased the membrane resistance and reduced the rectifying properties of the membrane. Only at 20 mM a small active response could be induced by outward current pulses. Low concentrations of noradrenaline (less than 10(-8) M) hyperpolarized the membrane, while higher concentrations (greater than or equal to 10(-7) M) depolarized. Isoprenaline at concentrations below 10(-7) M also hyperpolarized, but it depolarized from 10(-5) M onward. Acetylcholine at concentrations over 10(-8) M hyperpolarized the cells without exerting an effect on the resting tension, but it reduced a noradrenaline induced contraction. Low concentrations of caffeine (less than or equal to 2 mM) hyperpolarize the membrane, while higher concentration (greater than or equal to 5 mM) depolarize. Caffeine is found to be a more efficient releaser of cellular Ca than noradrenaline. This might be due to the weak beta-agonist action of noradrenaline appearing at high noradrenaline concentrations. The hypothesis is supported by the finding that a beta-stimulation increases the Ca-uptake in the intracellular store. The study of the electrophysiological effects of different stimuli do not suggest an important role for electromechanical coupling in this tissue.