Characterization of macroscopic outward currents of canine colonic myocytes

Abstract

Outward currents of colonic smooth muscle cells were characterized by the whole cell voltage-clamp method. Four components of outward current were identified: a time-independent and three time-dependent components. The time-dependent current showed strong outward rectification positive to -25 mV and was blocked by tetraethylammonium. The time-dependent components were separated on the basis of their time courses, voltage dependence, and pharmacological sensitivities. They are as follows. 1) A Ca2+-activated K current sensitive to external Ca2+ and Ca2+ influx was blocked by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (0.1 X 10(-3) M) and nifedipine (1 X 10(-6) and was increased by elevated Ca2+ (8 X 10(-6) M) and BAY K 8644 (1 X 10(-6) M). 2) A "delayed rectifier" current was observed that decayed slowly with time and showed no voltage-dependent inactivation. 3) Spontaneous transient outward currents that were blocked by ryanodine (2 X 10(-6) M) were also recorded. The possible contributions of these currents to the electrical activity of colonic muscle cells in situ are discussed. Ca2+-activated K current may contribute a significant conductance to the repolarizing phase of electrical slow waves.

Outward Currents in Smooth Muscle Cells Isolated from Sheep Mesenteric Lymphatics

“…It seems possible that the inactivation may reflect the inactivation of Ca¥ current (e.g. Cole & Sanders, 1989). Iberiotoxin (300 nÒ) reduced these currents by approximately 90% throughout the voltage range ( Fig.…”

Main Components of Voltage-Sensitive K⁺ Currents of the Human Colonic Smooth Muscle Cells

“…In the canine colonic circular muscle, which has been the most extensively studied, the activation o f a fast volt age-sensitive K + current was suggested to limit the up stroke depolarization [4], Others reported the existence o f multiple components o f the so-called delayed rectifier K + current, suspected to determine the amplitude and dura tion o f the plateau potential [3,5]. Although the C a 2+ sen sitivity o f K + channels in the canine colonic smooth mus cle was investigated at the single-channel level [6], the results obtained were not comparable to those from the macroscopic currents [5], and did not include studies focused on the C a 2+ dependence o f voltage-sensitive K + channels.…”

“…Although the C a 2+ sen sitivity o f K + channels in the canine colonic smooth mus cle was investigated at the single-channel level [6], the results obtained were not comparable to those from the macroscopic currents [5], and did not include studies focused on the C a 2+ dependence o f voltage-sensitive K + channels.…”

Sodium Channel Blockers Alter Slow-Wave Frequency of the Rat Stomach in vivo

“…In both cases, the resting membrane potential decreases; there fore, the changes in slow waves induced by ouabain may be due to depolarization inacti vating other voltage-dependent mechanisms which are controlling slow-wave activity [14]. In a series of experiments, the ionic mechanisms of these cyclic membrane po tential changes have recently been character ized in colonic circular smooth muscle [5,6,19,20]: at resting membrane potential, vol tage-dependent calcium channels were acti vated causing a depolarization of the cell; this depolarization activates voltage and cal cium-dependent potassium channels leading to repolarization. The activation and inacti vation characteristics of these ion channels induce a window current resulting in the pla teau phase of the slow waves [6].…”