Whole-cell recordings of ionic currents in bovine somatotrophs and their involvement in growth hormone secretion

Abstract

1. The whole-cell mode of the patch-clamp technique was used to record voltage-activated cationic currents in immunocytochemically identified bovine somatotrophs. 2. In current-clamp mode, cells had a resting membrane potential of -83.0 +/- 4.5 mV, and an input resistance of 8.4 +/- 2.1 G omega. Cells rarely fired action potentials spontaneously, but fired one to three action potentials in response to a suprathreshold current pulse. 3. Under voltage clamp, in Ca2+-free media, the action potential was shown to be composed of a TTX-sensitive inward Na+ current and an outward K+ current. 4. The isolated Na+ current had a threshold of approximately -50 mV, and rapidly activated and then inactivated to a small steady-state current. Peak Na+ current amplitude with 140 mM-external Na+ was 341.1 +/- 33.5 pA (n = 14) at a membrane potential of -32.1 +/- 2.4 mV (n = 14). 5. With Ca2+ or Ba2+ (5-30 mM) as the only membrane-permeable cation, voltage pulses to potentials more positive than -55 mV from a holding potential of -80 mV revealed a rapidly activating current component that was followed by a second, very slowly inactivating, current component, most clearly seen with Ba2+. Both components were maximally activated between 0 and +10 mV, were TTX insensitive, but were blocked by 4 mM-Co2+. 6. Three components of the isolated K+ current were identified (IA, IK and IK(Ca] by their voltage sensitivity, Ca2+ dependence and their response to 4-aminopyridine (4-AP) and tetraethylammonium (TEA). 7. Growth hormone (GH)-releasing hormone (GHRH) applied to cells under whole-cell voltage clamp had no effect on either steady-state or voltage-activated ionic currents. This is probably due to dialysis of cytoplasmic compounds vital for GHRH activation of the cell. 8. Both basal and GHRH-stimulated GH secretion were unaffected by TTX, implying that the Na+ action potential is not critical for such release. In contrast the Ca2+ channel blocker Co2+ attenuated GH release in both cases. The K+ channel blocker TEA stimulated GH release above basal and GHRH-stimulated levels.