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Antagonistic effect of amiloride on alpha-2 adrenoceptor-mediated pigment aggregation: pharmacological heterogeneity between B-HT 920 and noradrenaline

SP Svensson, LG Martensson, N Grundstrom, RG Andersson, EJ Cragoe and JO Karlsson

Department of Pharmacology, Linkoping University, Sweden.

Pigment aggregation in melanophores from the cuckoo wrasse (Labrus ossifagus L.) has previously been shown to be mediated by alpha-2 adrenoceptors. In the present investigation, the effect of amiloride on pigment aggregation induced by noradrenaline and B-HT 920 was studied. Amiloride caused a parallel shift to the right in concentration- response curves for 5-allyl-2-amino-5,6,7,8-tetrahydrothiazolo-[4,5- d]azepine-dihyd roc hloride: (B-HT 920), with no change in the maximal response, indicating simple competitive inhibition. Subsequent Schild plot analysis gave a straight line with a slope of almost unity (0.95) and a KB of 5 x 10(-6) M. In contrast to the effect of amiloride on B- HT 920-induced pigment aggregation, only a very high concentration of amiloride (1 x 10(3) M) inhibited the corresponding effect of noradrenaline. Na+/H+ exchange has been suggested as a possible mechanism for alpha-2 adrenoceptors, and the amiloride analogs 5-(N,N- dimethyl)amiloride and 5-(N,N-hexamethylene)amiloride are known to be more specific than amiloride itself in this respect. However, these analogs inhibited the effect of B-HT 920 significantly less effectively than amiloride. Furthermore, manipulating the extracellular pH between 6.8 and 7.8 did not affect the concentration response curves of B-HT 920. Neither did the prostanoid pathway inhibitors, quinacrine or indomethacin, inhibit the pigment-aggregating effect of B-HT 920. The present results suggest that amiloride competes with B-HT 920 as receptor antagonist, and that Na+/H+ exchange is insignificant for the alpha-2 adrenoceptor-stimulated pigment aggregation.(ABSTRACT TRUNCATED AT 250 WORDS)

Volume 258, Issue 2, pp. 447-451, 08/01/1991
Copyright © 1991 by American Society for Pharmacology and Experimental Therapeutics






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Copyright © 1991 by the American Society for Pharmacology and Experimental Therapeutics.