Potassium channels of myenteric neurons in guinea-pig small intestine

Patch-clamp recording was used to study rectifying K+ currents in myenteric neurons in short-term culture. In conditions that suppressed Ca2+-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,I-dr) and inwardly rectifying current (I-K,I-ir). I-K,I-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,I-ir did not inactivate. I-K,I-ir was abolished in K+-free solution. Increases in external K+ increased I-K,I-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,I-ir was suppressed by external Cs+ and Ba2+ in a concentration-dependent manner. Ca2+ and Mg2+ were less effective than Ba2+. I-K,I-ir was unaffected by tetraethylammonium ions. I-K,I-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,I-dr was suppressed in a concentration-, but not voltage-dependent manner by either tetraethylammonium or 4-aminopyridine and was insensitive to Cs+. The results suggest that I-K,I-ir may be important in maintaining the high resting membrane potentials found in afterhyperpolarization-type enteric neurons. They also suggest importance of I-K,I-ir channels in augmentation of the large hyperpolarizing after-potentials in afterhyperpolarization-type neurons and the hyperpolarization associated with inhibitory postsynaptic potentials. I-K,I-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 Ca2+-activated K+ current. (C) 1998 IBRO. Published by Elsevier Science Ltd.