Characterization of the cyclic AMP-independent actions of somatostatin in GH cells. II. An increase in potassium conductance initiates somatostatin-induced inhibition of prolactin secretion.
The neuropeptide somatostatin inhibits prolactin release from GH4C1 pituitary cells via two mechanisms, inhibition of stimulated adenylate cyclase activity and an undefined cAMP-independent process. Somatostatin also hyperpolarizes GH4C1 cells and reduces their intracellular free Ca2+ concentration ([Ca2+]i) in a cAMP-independent manner. To determine whether these ionic changes were involved in the cAMP-independent mechanism by which somatostatin inhibited secretion, changes in cAMP levels were prevented from having any biological consequences by performing experiments in the presence of a maximal concentration of a cAMP analog. Under these conditions, inhibition of prolactin release by somatostatin required a transmembrane concentration gradient for K+ but not one for either Na+ or Cl-. However, elimination of the outward K+ gradient did not prevent somatostatin inhibition of vasoactive intestinal peptide-stimulated hormone release. Therefore, somatostatin's cAMP-mediated mechanism does not require a K+ gradient, whereas its cAMP-independent inhibition of secretion appears to result from a change in K+ conductance. Consistent with this conclusion, membrane hyperpolarization with gramicidin (1 microgram/ml) mimicked somatostatin inhibition of prolactin release. In addition, the K+ channel blocker tetrabutylammonium prevented the effects of somatostatin on the membrane potential, the [Ca2+]i and hormone secretion. Nonetheless, a K+ gradient was not sufficient for somatostatin action. Even in the presence of a normal K+ gradient, somatostatin was only able to inhibit prolactin release when the extracellular Ca2+ concentration was at least twice the [Ca2+]i. Furthermore, the calcium channel blocker, nifedipine (10 microM), which prevents the action of somatostatin to reduce the [Ca2+]i, specifically blocked inhibition of prolactin release via somatostatin's cAMP-independent mechanisms. Therefore, a decrease in Ca2+ influx through voltage-dependent Ca2+ channels produces both the fall in [Ca2+]i and inhibition of hormone secretion in response to somatostatin.
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000494:pharmacology / Q000502:physiology*; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D011388:Prolactin / Q000378:metabolism*; D013004:Somatostatin / Q000494:pharmacology*; D013873:Thionucleotides / Q000494:pharmacology
D013873:Thionucleotides; C015929:8-((4-chlorophenyl)thio)cyclic-3',5'-AMP; D013004:Somatostatin; D011388:Prolactin; D000242:Cyclic AMP; D011188:Potassium; D002118:Calcium