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

Abstract

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.