Effects of Various Secretagogues Upon 42K and 22NA Uptake During in Vitro Hormone Release from the Rat Adenohypophysis

1. The in vitro uptake of (22)Na and (42)K was measured simultaneously in rat adenohypophyses during hormone release produced by several secretagogues and during inhibition of hormone release in Ca-free media.2. Intracellular adenohypophysial [Na(+)] and [K(+)] changed only slightly when the uptake changed. This would indicate that relative permeability changes were the primary effect of the treatments.3. The uptake of (42)K was increased by elevated external [K(+)], but was unaffected by the presence or absence of Ca(2+). Acid extracts of hypothalamus-stalk-median eminence or cerebellum also increased the (42)K uptake.4. The uptake of (22)Na or (24)Na was decreased by elevated [K(+)]. Uptake was increased in Ca-free Krebs-Ringer bicarbonate; but was unaltered when [K(+)] was concurrently increased.5. Neither purified growth hormone releasing hormone, synthetic lysine-vasopressin, dibutyryl cyclic AMP nor theophylline had an effect on the uptake of either K(+) or Na(+).6. The rapid uptake of (22)Na and its smaller volume of distribution compared to absolute measurements of intracellular [Na(+)] suggest that the plasma membrane of adenohypophysial cells is relatively impermeable to Na(+).7. We conclude that changes in the uptake of Na(+) and K(+) associated with hormone release are incidental to the release process.8. Hormone release produced by elevated external [K(+)] is most likely due to a non-specific increase in permeability of the cell membranes, facilitating Ca(2+) entry into the cytoplasm.9. The results suggest that the low resting transmembrane potentials of adenohypophysial cells may be due to their conjoint relatively high permeability to both K(+) and Ca(2+), rather than K(+) and Na(+).