Action potentials induced in guinea pig arterial smooth muscle by tetraethylammonium

Abstract

The membrane potential (Em) of an isolated small artery, namely, the superior mesenteric artery of the guinea pig, was measured by intracellular microelectrodes. When impaled with microelectrodes, the arterial smooth muscle cells gave a mean resting potential of -54 mV. The mean input resistance was 8.5 M.OMEGA.. The cells were quiescent and electrically inexcitable. Addition of tetraethylammonium ion (TEA, 5 mM) induced production of large overshooting action potentials upon electrical stimulation. The amplitude and maximal rate of rise (+.ovrhdot.Vmax) of these action potentials increased as a function of external Ca concentration ([Ca]o), and the action potentials were blocked by verapamil or Mn2+ (1 mM). The slope of the action potential amplitude vs. log [Ca]o curve was 29 mV/decade (from 0.5-4.8 mM Ca2+). Neither the amplitude nor +.ovrhdot.Vmax of the action potential was significantly altered in low Na+ solutions. Tetrodotoxin (TTX) had no effect on the action potentials. When +.ovrhdot.Vmax was measured as a function of Em, half-maximal inactivation of the inward current channels occurred at -47 mV, and complete inactivation occurred at -22 mV. The inward current of the TEA-induced action potential is carried solely by Ca2+ through TTX-insensitive, verapamil-sensitive slow channels.