Subpopulations of Somatotropes with Differing Intracellular Calcium Concentration Responses to Secretagogues

Abstract

Multiple secretagogues stimulate the release of growth hormone (GH). The present studies examined the ability of chicken somatotropes to respond to GH secretagogues with increased intracellular calcium concentrations ([Ca²⁺]_i). It was hypothesized that there are subsets of the somatotrope population with different responsiveness to the various secretagogues. Somatotropes were identified and distinguished from other adenohypophyseal cells, by their unique ability to respond to GH-releasing hormone with increased [Ca²⁺]_iwith immunocytochemistry used as a post-hoc confirmatory test. Large increases in [Ca²⁺]_i (222 ± 16 n M) were evoked by thyrotropin-releasing hormone in only 73% of the somatotropes. Similarly, [Ca²⁺]_i was increased by perifusion with pituitary adenylate cyclase-activating peptide in 85% and by leptin but only in 51% of somatotropes. Ghrelin acutely increased [Ca²⁺]_i in only 21% of somatotropes. Perfusion with gonadotropin-releasing hormone elevated [Ca²⁺]_i, but in only 40% of somatotropes. The kinetics of calcium transients and the magnitude of the response differed from those observed in the presumptive gonadotropes. It is concluded that there are subsets of the somatotrope population in the anterior pituitary gland with differences in their ability to respond to various secretagogues.