A differential effect of trypsin on pituitary gonadotropin‐releasing hormone receptors from intact and ovariectomized rats

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Abstract
Pituitary receptors for gonadotropin-releasing hormone generated following ovariectomy are susceptible to the action of trypsin while those present in intact metestrous females are not. The action of trypsin of gonadotropin-releasing hormone receptors in pituitary cells derived from ovariectomized rats was dependent on time and dose. Trypsin exerted a maximal effect of 40-50% reduction without causing full abolition of the binding activity, indicating that 2 populations of gonadotropin-releasing hormone receptors can be differentiated in pituitaries derived from ovariectomized rats: one which is sensitive to limited proteolysis by trypsin and another which is resistant to it. Scatchard analysis of gonadotropin-releasing hormone antagonist binding to homogenate of trypsin-treated hemipituitaries derived from ovariectomized rats revealed that trypsin reduced receptor density while slightly increasing binding affinity. The in vitro responsiveness of hemipituitaries from ovariectomized rats to gonadotropin-releasing hormone and to K+-induced depolarization was reduced following trypsin treatment, while that of hemipituitaries derived from metestrous females was not affected or increased. Trypsin reduced the maximal response level by 30-50% but did not modify the median effective dose of gonadotropin-releasing hormone (1 nM). The reduced gonadotropin-releasing hormone receptor density and the lower responsiveness caused by trypsin in pituitary tissue from ovariectomized rats are not due to a selective loss of gonadotrophs, as trypsin treatment did not affect the lutropin content of the glands. Two populations of gonadotropin-releasing hormone receptors can be distinguished on the basis of their susceptibility or resistance to limited proteolysis by trypsin and apparently these 2 gonadotropin-releasing hormone receptor populations are reciprocally affected by ovariectomy.