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

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Abstract
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 77mM. 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 (−8 M) hyperpolarized the membrane, while higher concentrations (≥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 (≤2 mM) hyperpolarize the membrane, while higher concentration (≥5 mM) depolarize. Caffeine is found to be a more efficient releaser of cellular Ca than noradrenaline. This might be due to the weak β-agonist action of noradrenaline appearing at high noradrenaline concentrations. The hypothesis is supported by the finding that a β-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.