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 damp condition an outward K+ current, a tetrodotoxinsensitive 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 KV.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 Ca21 currents. Finally, we discuss other possibilities for the inhibitory effect of somatostatin on GH secretion.Somatostatin (somatotropin release-inhibiting factor, SRIF) is a potent inhibitor of growth hormone (GH) secretion. It also inhibits the secretion of other pituitary hormones (1, 2) and extrapituitary hormones (3). GH secretion is known to be regulated by the level of intracellular cyclic AMP (cAMP), which is raised by a stimulatory hormone, such as GHreleasing factor (4). It has been claimed that somatostatin exerts its inhibitory action through the reduction of cAMP production (5-7).Ca2l is another important factor that controls GH secretion from the anterior pituitary. It has been shown that Ca2l is required for GH release evoked by high concentrations of extracellular K+ (8) and GH-releasing factor (4, 9). Various anterior pituitary cell lines have b*,n shown to have action potentials that are dependent on Ca2l or Na+ (10). The influx of Ca2l is postulated to be controlled at least partly by the action potential. The membrane-signal transduction mechanism for somatostatin action has not yet been fully clarified in the pituitary. In the rat pancreas islet it has been reported that somatostatin activates a K+ conductance and inhibits glucose-induced insulin release (11). In analogy, a similar mechanism of somatostatin may exist in the pituitary. For the investigation of the mechanism of somatostatin action in the pituitary, human GH-producing pituitary tumor cells are useful because the presence of somatostatin receptors in the cell membrane has been demonstrated (12...

The effect of pertussis toxin on somatostatin-induced K+ current was examined in dissociated human pituitary tumor cells obtained from two acromegalic patients. Somatostatin-induced hyperpolarization or K+ current was observed in 20 of 23 cells in adenoma 1 and 10 of 11 cells in adenoma 2. After treatment with pertussis toxin for 24 h, these responses were completely suppressed (0/14 in adenoma 1, 0/10 in adenoma 2). Spontaneous action potentials, K+, Na+, and Ca2+ currents were well preserved after pertussis toxin treatment. When crude membrane fraction was incubated with [32P]NAD, a 41K protein was ADP-ribosylated by pertussis toxin. Hormone release was inhibited by somatostatin and this inhibition was blocked by pertussis toxin treatment.

The role of GTP on somatostatin-induced K+ current increase was examined in dissociated human pituitary tumor cells obtained from three acromegalic patients. Pituitary cells in culture were voltage-clamped by using the patch clamp technique in the whole-cell configuration. Somatostatin (100 nM) increased the membrane permeability to K+ ions and inhibited hormone secretion. A current-voltage relation of the somatostatin-induced K+ current showed an inward rectification when the concentration of extracellular K+ ions was increased.

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Voltage-gated Ca2+ channel currents were examined in human growth hormone-producing cells using the whole cell variation of patch electrode voltage clamp. In 20 mM Ba2+, Na+-free medium, the inward current was composed of a fast-inactivating component (transient type) and a steady component (long-lasting type). These two types had different properties. The transient type inactivated with the depolarizing prepulse but the long-lasting type did not. The transient type was activated at more negative potential levels than the long-lasting type. Deactivation kinetics of the transient type were slower than that of the long-lasting type. Nitrendipine inhibited and Bay K 8644 enhanced Ca2+ channel currents, and the effects of these agents were more prominent on the long-lasting type than on the transient type. Hormone release was inhibited by nitrendipine and enhanced by Bay K 8644, indicating involvement of Ca2+ influx through voltage-gated channels in the release process.