Ontogeny of adrenoceptor-mediated contractile responses of Müller's smooth muscle in the spontaneously hypertensive rat: role of impulse activity.

Contractile responses of Müller's smooth muscle to alpha-adrenoceptor stimulation were evaluated in anesthetized Spontaneously Hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats on postnatal days 1-80. Force of contraction elicited by methoxamine, tyramine and electrical stimulation of the preganglionic nerve in SHRs was significantly less than that of WKY rats throughout development. alpha-Adrenoceptor sensitivity, as evaluated by methoxamine ED50, was comparable at 10 and 20 days of age, but receptor sensitivity was decreased in SHRs at 40 and 80 days. Elimination of impulse activity by decentralization of the superior cervical ganglion on postnatal day 3 resulted in a decrease in force of contraction of about 25% in both strains, and responses in SHRs remained significantly below those of WKY rats. Decentralization increased adrenoceptor sensitivity in both groups, and differences between strains were eliminated. It is concluded that the smaller adrenoceptor-mediated contractions in SHRs are an intrinsic property of the muscle while adrenoceptor subsensitivity is a consequence of elevated sympathetic nerve activity in this strain.