Potassium channels of myenteric neurons in guinea-pig small intestine

Abstract

Patch-clamp recording was used to study rectifying K + currents in myenteric neurons in short-term culture. In conditions that suppressed Ca 2+-activated K + current, three kinds of voltage-activated K + currents were identified by their voltage range of activation, inactivation, kinetics and pharmacology. These were A-type current, delayed outwardly rectifying current (I K.dr) and inwardly rectifying current (I K.ir). I K.ir consisted of an instantaneous component followed by a time-dependent current that rapidly increased at potentials negative to −80 mV. Time-constant of activation was voltage-dependent with an e-fold decrease for a 31-mV hyperpolarization amounting to a decrease from 800 to 145 ms between −80 and −100 mV. I K.ir did not inactivate. I K.ir was abolished in K +-free solution. Increases in external K + increased I K.ir conductance in direct relation to the square root of external K + concentration. Activation kinetics were accelerated and the activation range shifted to more positive K + equilibrium potentials. I K.ir was suppressed by external Cs + and Ba 2+ in a concentration-dependent manner. Ca 2+ and Mg 2+ were less effective than Ba 2+ I K, ir was unaffected by tetraethylammonium ions. I K.dr was activated at membrane potentials positive to −30 mV with an e-fold decrease in time-constant of activation from 145 to 16 ms between −20 and 30 mV. It was half-activated at 5 mV and fully activated at 50 mV. Inactivation was indiscernible during 2.5 s test pulses. I K.dr was suppressed in a concentration-, but not voltage-dependent manner by either tetraethylammonium or 4-aminopyridine and was insensitive to Cs 2+. The results suggest that I K.ir may be important in maintaining the high resting membrane potentials found in afterhyperpolarization-type enteric neurons. They also suggest importance of I K.ir channels in augmentation of the large hyperpolarizing after-potentials in afterhyperpolarization-type neurons and the hyperpolarization associated with inhibitory postsynaptic potentials. I K.dr in afterhyperpolarization-type enteric neurons has overall kinetics and voltage behaviour like delayed rectifier currents in other excitable cells where the currents can also be distinguished from A-type and Ca 2+-activated K + current.