Growth hormone releasing factor evokes rhythmic hyperpolarizing currents in rat anterior pituitary cells.

Abstract

1. The effect of human pancreatic growth hormone releasing factor (hpGHRF) on the electrical activity of dissociated rat anterior pituitary cells in culture was studied, using both the cell-attached and whole-cell modes of the patch-clamp recording technique. 2. To avoid possible wash-out of the responses, extracellular records were made from cell-attached patches. Application of hpGHRF to the cells produced rhythmic inward currents through the patches, attributable to rhythmic hyperpolarizations of the cell membrane outside the patch. The amplitude of the current oscillations was 1-8 pA and the frequency 0.05-0.4 Hz. 3. Flooding the cells with K+ ions from a small pipette containing 50 mM or 100 mM-K+ resulted in a reversible attenuation or block of the rhythmic inward currents evoked by hpGHRF, indicating that changes in K+ conductance were involved in the responses. 4. Flooding the cells with a solution containing 10 mM-EGTA blocked these rhythmic inward currents reversibly, suggesting the involvement of Ca2+ in the responses. In addition, responses were blocked by adding Co2+ (5-10 mM) to the bathing medium. The presence of tetrodotoxin (3 microM) had no effect, ruling out the participation of voltage-gated Na+ channels. 5. With whole-cell recording, the resting potential (-41.46 +/- 7.78 mV) and input resistance (5.34 +/- 3.73 G omega) of anterior pituitary cells in culture were found to be similar to those previously reported for pituitary cells and chromaffin cells with the same recording method. 6. In whole-cell experiments, application of hpGHRF (shortly prior to intracellular penetration) evoked rhythmic outward currents, associated with conductance increases, when the cells were clamped at their resting potential. The persistence of these currents in the 'voltage-clamped' cell indicated that the rhythmicity was not related to voltage-dependent phenomena. The currents disappeared within 4 min after breaking into the cell, presumably because of 'washout' of cell constituents into the pipette. 7. The reversal potential (-60 mV) of the hpGHRF-induced currents was negative to the resting potential of the cells (-41 mV), further indicating that hpGHRF would evoke rhythmic hyperpolarizations in unclamped cells, possibly due to periodic increases in K+ conductance. 8. The possible relation of these rhythmic currents to hpGHRF-induced secretion of growth hormone is discussed.