Late slow outward currents following hyperpolarizing commands are associated with intracellular Ca2+ increase in guinea-pig submucous neurons: investigations with fura-2.

Intracellular recordings were made from submucous neurons in the guinea-pig isolated caecum. A hyperpolarization lasting several seconds followed the termination of a strong hyperpolarizing current pulse; this was always accompanied by a decrease in input resistance. In neurons voltage-clamped at -30 mV, a late slow outward current (LSOC) flowed for more than 10 s, following the termination of a hyperpolarizing command (-100 mV, 2 s). This current was dependent on extracellular K+ concentration, but was not affected by tetraethylammonium, D-tubocurarine and 4-aminopyridine, and was reversibly blocked by low calcium/high magnesium solution. Intracellular calcium concentration ([Ca2+]i) was measured by using a fluorescent calcium indicator, fura-2. In voltage-clamped neurons, [Ca2+]i decreased during the hyperpolarizing command pulse, but increased above the original level at the end of the command. The time course of the change in [Ca2+]i was almost the same as that of the LSOC. Intracellular injection of cesium ions depressed the LSOC, but did not affect the change in [Ca2+]i. The present results indicate that the LSOC is due to activation of calcium-sensitive potassium conductance.