Modulation of the inactivation of voltage-dependent potassium channels by cAMP

Modulation of voltage-dependent potassium currents can alter the shape and timing of action potentials, thereby altering neurotransmitter release. To examine the effect of a cAMP analog on potassium currents in metabolically intact cells, perforated-patch and cell-attached patch recordings were carried out using the GH4C1 pituitary cell line. A major component of voltage-dependent potassium current in these cells inactivates slowly, with a time constant of several seconds. Application of dibutyryl cAMP decreased this current at voltages positive to -10 mV and increased the rate of inactivation by approximately twofold. Single channel recordings revealed two channel types whose voltage dependence and kinetics of inactivation match those of the macroscopic current. One of these, the smaller conductance (7.5 pS) channel, was sensitive to the cAMP analog, which decreased the latency of the channel to enter a long-lasting inactivated state. Ensemble averages of the activity of this channel showed that, consistent with its effect on the macroscopic current, dibutyryl cAMP increased its rate of inactivation. Somatostatin, an agent that is known to activate a serine/threonine phosphatase in these cells, completely reversed the effect of dibutyryl cAMP on the channel, while the cyclic GMP analog, dibutyryl cyclic GMP was without effect. In contrast, the rate of inactivation of the larger conductance (approximately 19 pS) channel was not accelerated by dibutyryl cAMP. These studies indicate that different channel subtypes expressed in a single cell respond differently to elevations of cAMP, and suggest that the overall response of potassium currents to second messengers may be determined by the ratio of different channel subtypes.