Influence of the lonic Environment on Spontaneous Electrical and Mechanical Activity of the Rat Portal Vein

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Abstract
The effects of variations in external K+ and Ca2+ concentrations on electrical and mechanical activity of the isolated rat portal vein were studied using a sucrose-gap technique. Spontaneous activity in normal Krebs solution consists of bursts of action potentials accompanied by phasic contractions. Moderate reduction in [K+]o results in slight hyperpolarization with decreased discharge frequency. Greater reduction or total removal of extracellular K causes a temporary increase in burst frequency. Graded increases in [K+]o from 6 up to 128 mM cause depolarization with an increase in discharge frequency and in contraction amplitude until [K+]o reaches a level of 6 to 7 times normal when spike potentials are abolished and a contracture develops. The relationship between membrance potential and log [K+]o indicates a relatively low permeability to K+. Increasing [Ca2+]o causes slight hyperpolarization, decreased burst frequency, increased burst duration and increased contraction amplitude. Reducing [Ca2+]o causes increased frequency of discharge and shortening of bursts, until single spikes appear, each preceded by a prepotential; thus the activity resembles that in cardiac pacemaker cells. In Ca2+-free solution, electrical and mechanical activity ceases within 10 to 15 min. Calcium is essential for activation of the contractile mechanism and for maintenance of spontaneous spike discharge.