Sodium and calcium action potentials in human anterior pituitary cells

Rat lactotrophs isolated by fluorescence-activated cell sorting are electrically excitable

Lactotrophs (prolactin-containing cells) from the anterior pituitary of the adult female rat were labeled by a cell-surface reaction with anti-prolactin antibodies and then isolated by fluorescence-activated cell sorting (FACS). FACS-isolated pituitary cells were maintained in culture 1-9 days, after which their excitable membrane properties were examined using the whole-cell patch recording technique. Comparisons between lactotrophs sorted at different laser-power settings on the FACS showed that although the electrical excitability of the sorted lactotrophs was acutely altered by exposure to high laser power, excitability comparable to unsorted cells was retained by the use of minimal laser power (10 mW). Recordings were made from 67 cells that had been isolated using the low laser power. These cells had resting membrane potentials ranging from -22 mV to -60 mV. More than 80% of the cells responded to depolarizing current injections with regenerative, full-amplitude, overshooting action potentials. Approximately 20% of these cells exhibited spontaneous 20-30 mV fluctuations in the level of resting membrane potential, the majority of which did not overshoot 0 mV. Both the action potentials and the spontaneous fluctuations involved Na+ and Ca2+ ion conductance mechanisms.

Hyperpolarization of the membrane potential caused by somatostatin in dissociated human pituitary adenoma cells that secrete growth hormone

Membrane electrical properties and the response to somatostatin were examined in dissociated human pituitary adenoma cells that secrete growth hormone (GH). Under current clamp condition with a patch electrode, the resting potential was -52.4 +/- 8.0 mV, and spontaneous action potentials were observed in 58% of the cells. Under voltage clamp condition an outward K+ current, a tetrodotoxin-sensitive Na+ current, and a Ca2+ current were observed. Cobalt ions suppressed the Ca2+ current. The threshold of Ca2+ current activation was about -60 mV. Somatostatin elicited a membrane hyperpolarization associated with increased membrane permeability in these cells. The reversal potential of somatostatin-induced hyperpolarization was -78.4 +/- 4.3 mV in 6 mM K+ medium and -97.2 +/- 6.4 mV in 3 mM K+ medium. These reversal potential values and a shift with the external K+ concentration indicated that membrane hyperpolarization was caused by increased permeability to K+. The hyperpolarized membrane potential induced by somatostatin was -63.6 +/- 5.9 mV in the standard medium. This level was subthreshold for Ca2+ and Na+ currents and was sufficient to inhibit spontaneous action potentials. Hormone secretion was significantly suppressed by somatostatin and cobalt ions. Therefore, we suggest that Ca2+ entering the cell through voltage-dependent channels are playing an important role for GH secretion and that somatostatin suppresses GH secretion by blocking Ca2+ currents. Finally, we discuss other possibilities for the inhibitory effect of somatostatin on GH secretion.

Different sodium requirements for 86Rb efflux and for growth hormone and prolactin secretion from bovine anterior pituitary cells

The sodium dependence of growth hormone and prolactin secretion and of 86Rb efflux from bovine anterior pituitary cells in response to acetylcholine and TRH was examined. Decreasing the external sodium concentration prevented the increases in the rates of 86Rb efflux and of growth hormone secretion caused by acetylcholine, or by TRH in the presence of IBMX. The growth hormone secretory response was less sensitive to sodium removal than was 86Rb efflux. However, even complete removal of extracellular sodium did not affect TRH-induced prolactin secretion. Ouabain and low extracellular potassium, which inhibit the sodium pump and increase intracellular sodium, prolonged the secretion of growth hormone in response to acetylcholine, but TRH-induced prolactin secretion was not affected. Inhibition of the sodium pump speeded the decline in the 86Rb efflux rate following stimulation by both acetylcholine and TRH. The results suggest that a sodium-dependent step is necessary for the efflux of 86Rb and for growth hormone secretion but not for prolactin secretion. The possible relationship between 86Rb efflux and hormone secretion from lactotrophs and somatotrophs is discussed.

An electrophysiological study of cultured human pituitary cells

The electrophysiological properties of tumoral pituitary cells were studied in 4 types of human adenomas including prolactinomas, growth-hormone-secreting tumors, adrenocorticotropin-hormone-secreting adenoma and 'non-functioning' tumors. Only 9% of the cells from prolactinomas and ACTH tumors were excitable but they never elicited spontaneous action potentials. These cells did not respond to substances known to act on the hormone-releasing process (thyreoliberin, dopamine). However, 37% of the cells cultured from growth-hormone-secreting adenomas and from 'non-functioning' tumors displayed action potentials. The action potential was calcium-dependent, i.e., it was blocked by cobalt, nickel and methoxyverapamil and could be recorded in a sodium-free medium. Thyreoliberin triggered action potentials, whereas dopamine and gamma-aminobutyric acid inhibited electrical activity. These results show that human tumoral pituitary cells in culture are able to generate Ca2+-dependent action potentials. The data from growth-hormone-secreting tumors are in good agreement with the stimulus-secretion coupling concept; however, differences in the response of cells cultured from other types of human pituitary tumors suggest that each type of adenoma has specialized membrane properties.