SOMATOSTATIN INHIBITS (D-ARG(6), PRO(9)-NET) SALMON GONADOTROPIN-RELEASING HORMONE-STIMULATED AND DOPAMINE D-1-STIMULATED GROWTH-HORMONE RELEASE FROM PERIFUSED PITUITARY-CELLS OF CHINESE GRASS CARP, CTENOPHARYNGODON IDELLUS

In this study, a heterologous radioimmunoassay (RTA) for grass carp GH has been validated and used to monitor the kinetics of GH release fro m perifused grass carp pituitary cells, To establish the anatomical sp ecificity of GH antiserum used in this RIA, immunohistochemical staini ng was performed in grass carp pituitary sections. Somatotrophs recogn ized by this GH antiserum were located mainly in the proximal pars dis talis without overlapping with gonadotrophs located in the same area o r with lactotrophs located in the rostral pars distalis. The immunorea ctivity of somatotrophs was abolished by preabsorbing GH antiserum wit h purified grass carp GH, suggesting that the possibility of a cross-r eactivity of antiserum with other grass carp pituitary hormones is unl ikely. Using I-125-labeled carp GH as the RIA tracer, parallelism was observed among the displacement curves of grass carp GH standard, gras s carp serum, and culture medium conditioned by grass carp pituitary c ells, suggesting that this RIA can be used to quantitate grass carp GH levels in biological samples. Using an in vitro column perifusion sys tem, a superactive gonadotropin-releasing hormone (GnRH) analog (D-Arg (6), Pro(9)-NEt)-sGnRH (sGnRHa, 0.3-30 nM), dopamine (DA, 0.1-10 mu M) , and the nonselective DA agonist apomorphine (0.1-10 mu M) stimulated GH release from grass carp pituitary cells in a dose-dependent manner , The GH-releasing effect of DA was mimicked by the D-1 agonists SKF38 393 (0.1-10 mu M) and SKF77434 (0.1-10 mu M), but not by the D-2 agoni st LY171555 (3 mu M). In addition, the GH response to DA (1 mu M) was blocked by the D-1 antagonist SCH23390 (5 mu M) but not by the D-2 ant agonist (+/-) sulpiride (5 mu M), suggesting that the GH-releasing act ion of DA is mediated through receptors resembling mammalian D-1 recep tors. Somatostatin-14-(SRIF14, 0.01-100 nM), unlike sGnRHa and DA, ind uced a dose-dependent suppression on basal GH release. At a high dose (100 nM), SRIF14, also abolished the GH responses to sGnRHa (100 nM), DA (10 mu M), and the D-1 agonist SKF38393 (3 mu M). These results, as a whole, provide evidence that GH release in the grass carp is under the direct regulation of GnRH, DA, and SRIF at the pituitary cell leve l, The present study also suggests that DA D-1 receptors are present i n grass carp pituitary cells mediating the GH-releasing action of DA. (C) 1998 Academic Press.