Electrical activity of single vascular smooth muscle fibers

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Abstract
Electrical activity of the longitudinal muscle layer in guinea pig excised superior mesenteric vein was recorded intracellularly. Bursts of action potentials were associated with spontaneous contractions. The resting potential between bursts ranged from 41 to 62 mv. Action potentials were accompanied by a slow component with an average amplitude of 25 [plus or minus] 1. 8 mv. The action potentials ranged from 35 to 59 mv and had a fast repolarization phase followed by marked post-hyperpolarization. Overshoot of a few millivolts was observed occasionally. Adrenaline [epinephrine] prolonged the bursts of spike discharges or initiated repetitive firing. High concentrations produced rapid membrane depolarization associated with increasing discharge frequency of action potentials of progressively smaller amplitudes. Acetylcholine had an excitatory effect but the depolarization was usually preceded by a slight hyperpolarization. The hyperpolarizing action of acetylcholine was more prominent when the initial membrane potential was low. Hyperpolarization could not be discerned when the initial membrane potential was low. Hyperpolarization could not be discerned when it was higher than 60 mv at the time of drug application. Asynchronous or partial excitation occurred independently along the muscle. Ionic mechanisms are discussed.