α1-Adrenergic stimulation of in vitro growth hormone release and cytosolic free Ca2+ in rat somatotrophs

The effects of α₁-adrenergic agents on GH release and intracellular free Ca²⁺ concentration ([Ca²⁺]i) were investigated in purified rat somatotroph preparations. Phenylephrine (PHE) stimulated in vitro GH release; the maximal effect (2.5-fold stimulation) occurred at 1 αM PHE. The effect was completely blocked by the α-adrenergic antagonist phentolamine and partially counteracted by the β-antagonist propranolol. Experiments with the fluorescent Ca²⁺ probe fura 2 show that PHE causes [Ca²⁺]i to rise from 178 ± 31 nM (mean ± SE; n = 25) to 370 ± 55 nM (n = 9). This effect was complete within 20 sec and was maintained for at least 5-10 min. The rise was rapidly interrupted by administration of 1 μM phentolamine. The β-receptor agonist isoproterenol caused a small [Ca²⁺]i rise due to action on α₁-adrenoreceptors. The PHE-induced [Ca²⁺]i rise showed two components: an initial peak due to Ca²⁺ mobilization from intracellular stores and a subsequent rise due to Ca²⁺ influx from the extracellular space. Somatostatin (SRIF) lowered both resting [Ca²⁺]i and Ca²⁺ influx stimulated by PHE. Pertussis toxin pretreatment did not modify PHE-induced [Ca²⁺]i changes, while it completely prevented the effect of SRIF on both resting and triggered [Ca²⁺]i, thus suggesting that a GTP-binding protein sensitive to the toxin is involved in the transduction of SRIF action. The increase in cAMP induced by cholera toxin pretreatment modified neither PHE nor SRIF action on [Ca²⁺]i. In conclusion on [Ca²⁺]i. In conclusion, in rat somatotrophs Ca²⁺ mobilization and influx are stimulated by α₁-adrenergic agents, and this triggered [Ca²⁺]i rise results in a stimulation of GH release. In these cells SRIF is able to reduce both resting [Ca²⁺]i levels and [Ca²⁺]i increases induced by α₁-adrenergic activation.