Mechanism of action of vasoactive intestinal polypeptide on myometrial smooth muscle of rabbit and guinea-pig.

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1. The action of vasoactive intestinal peptide (VIP) on the electrical and mechanical activity of strips of longitudinal myometrial smooth muscle from rabbits and guinea-pigs treated with oestradiol was studied in the sucrose-gap apparatus. 2. In myometrial strips which spontaneously exhibited regular contractions, or which were induced to contract rhythmically to the application of oxytocin, VIP reduced both the frequency and the force of contraction. 3. Contractions were associated with bursts of action potential discharge. In guinea-pig, the membrane potential reached its most negative value shortly following a burst and a slow decay of negativity followed ("generator potential'). VIP inhibited the decay of this negativity and increased the duration of the period between bursts. In rabbit myometrical strips, electrical discharges occurred less regularly but VIP also had an inhibitory action. The inhibitory action of VIP was not affected by the beta-adrenoreceptor blocker propranolol, by tetrodotoxin, or by apamin. 4. Using the double sucrose-gap apparatus, bursts of action potentials and contractions were elicited with depolarizing electrical pulses in the absence of oxytocin. Changes in membrane resistance were also estimated by eliciting hyperpolarizing electrotronic potentials. VIP hyperpolarized the membrane and inhibited contractions as depolarizing pulses now failed to reach threshold for action potential discharge or fewer action potentials were discharged. A small (about 10%) reduction in membrane resistance was freqeuently observed during the hyperpolarization. 5. If a single action potential was elicited in the presence of VIP, the tension generated by the muscle was less than in its absence. 6. In a calcium-free high-potassium (126 mM) solution, readmitting calcium produced contraction; VIP inhibited this contraction. Activation of beta-receptors by means of isoprenaline had a similar effect but unlike isoprenaline the action of VIP was not blocked by propranolol. 7. It is suggested that the primary action of VIP is on the calcium economy of the myometrial smooth muscle cell, possibly to accelerate sequestration and/or extrusion of calcium from the cell. In some way this is associated with inhibition of the generator potential, hyperpolarization, and with a small increase in permeability of the membrane to potassium.

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