Synthesis and secretion of growth hormone in the rat anterior pituitary. II. Properties of the isolated growth hormone storage granules.

Partially purified storage granule fractions obtained from rat anterior pituitaries have been utilized to study some properties of the isolated growth hormone granules during incubation in vitro. The isolated granules showed optimal stability at room temperature in the pH range 5-5-6-5 and dissolved progressively with time at a rate which was unaffected by the presence of EDTA, EGTA, ouabain, magnesium ions, or by resuspension in hypotonic media. However, the granules were partially stabilized by the presence of calcium, tyramine or adenosine phosphates; ATP was most effective of the agents tested and induced almost complete stability. Agents known to stimulate the secretion of growth hormone from the intact cells, cyclic 3',5'-AMP, dibutyryl cyclic 3',5'-AMP, theophylline or high potassium concentrations, were each without significant effect on the isolated granules. The granule fraction was ineffective in degrading added I-growth hormone over a pH range of 45-8-5 and was unable specifically to bind exogenous I-growth hormone. Further experiments to elucidate a possible cytological role of nucleotides in stabilization of growth hormone during storage granule formation demonstrated that adenine nucleotides were not constituents of the isolated granules, nor were they secreted into the incubation medium concomitantly with growth hormone. Depletion of intracellular ATP levels has previously been shown to prevent the formation of growthhormone storage granules, and it is suggested that one role of the nucleotide may be to facilitate the formation and stabilization of storage granules in the Golgi complex by interacting with the growth hormone to induce its precipitation.