The calcium-activated potassium conductance in guinea-pig myenteric neurones.

1. Intracellular recordings were made from guinea-pig myenteric neurones in vitro.2. From one to sixty action potentials were followed by an afterhyperpolarization, the amplitude and duration of which increased with the number of preceding action potentials.3. The afterhyperpolarization reversed its polarity at a membrane potential of -91 mV. This value changed by 58 mV when the potassium concentration of the perfusing solution was changed ten-fold.4. The afterhyperpolarization was abolished in calcium-free solutions. It was shortened in low calcium (1.2 mM) solutions and prolonged in solutions which contained high (5.0 mM) calcium concentrations, TEA (1 mM) or caffeine (1 muM).5. The conductance increase during the afterhyperpolarization (g(K, Ca)) was calculated from the amplitude of electrotonic potentials, taking advantage of the lack of membrane rectification in the range -60 to -90 mV. Peak g(K, Ca) increased as the number of action potentials was increased, but was relatively independent of membrane potential in this range.6. g(K, Ca) declined with a time course which was single exponential (time constant 1.5-5 s) following one to six action potentials, and double exponential (time constants about 3 and 12 s) following fifteen to sixty action potentials.7. It is concluded that the calcium which enters the neurone during the action potential elevates the membrane potassium conductance. The time course of this conductance increase probably reflects the free intracellular calcium concentration, and therefore describes the calcium sequestration or extrusion process.