ADRENALINE INDUCES HYPERPOLARIZATION IN FROG PITUITARY MELANOTROPHS THROUGH ACTIVATION OF POTASSIUM CHANNELS

A patch-clamp study was conducted on cultured frog pituitary melanotro phs, in order to investigate the effects of adrenaline on the electric al activity of these cells. In the whole-cell configuration, adrenalin e (1 mu M) caused hyperpolarization that was accompanied by a fall in membrane input resistance and a blockage of spontaneous action potenti als. Under voltage clamp, adrenaline elicited a net-outward current. T he hyperpolarization became undetectable at a command voltage of -100 mV which corresponded to the equilibrium potential of potassium ions. The effect of adrenaline on membrane potential and spontaneous activit y was blocked by the alpha(2)-adrenergic receptor antagonist yohimbine (1-10 mu M) but could not be mimicked by the alpha(2)-adrenergic agon ist clonidine(1-10 mu M). In the cell-attached configuration, exposure of the extra-patch membrane to adrenaline increased the occurrence of single-channel currents with a slope conductance of 100 pS. The deduc ed reversal potential of these currents corresponded to the equilibriu m potential of potassium ions. These results suggest that frog melanot rophs display an alpha(2)-adrenergic receptor subtype coupled to potas sium channels involved in hyperpolarization.