Involvement of postsynaptic adrenergic mechanism(s) in the ATP-contractions of the rat anococcygeus muscle and the effect of extracellular Ca2+.

Clonidine, ICI-106270 (6-aryl 2,367 tetrahydro 5-H-pyrrolo(1,2-a)-imidazole derivative), B-HT-920 (6-Allyl-2-amino-5,6,7,8-tetrahydro-4H-thiozolo = [4,5-d]azepine-dihydrochloride, and guanfacine, agonists at alpha 2-adrenoceptors, produced concentration-dependent contractions of the rat anococcygeus muscle. The order of potency of these agonists in producing the response was clonidine greater than ICI-106270 greater than guanfacine greater than B-HT 920. Yohimbine (10(-6) M) competitively antagonized the contractile response due to these agents acting at alpha 2-adrenoceptors. ATP (10(-6) M-10(-4) M) also produced concentration-dependent contractions of the rat anococcygeus muscle. Prazosin (10(-8) M) an alpha 1-blocker, significantly antagonized ATP-induced contractions in a competitive manner. ATP (10(-7) M) in a concentration that had no effect on basal tension of the preparation, tends to potentiate phenylephrine responses, though not significantly. The sensitivity of the anococcygeus muscle to ATP was enhanced after 24 hr (maximum catecholamine depletion period) of reserpine (5 mg/kg, i.p.) treatment. Yohimbine (10(-6) M), a selective alpha 2-blocker, significantly antagonized ATP-induced contractions and shifted the ATP concentration-response curve towards right. The antagonism was found to be competitive. The ATP-contractions were also sensitive to inhibition by reduced calcium contents (quarter or calcium free) in the Krebs bicarbonate solution. However, it was observed that ATP was capable of producing contractions of anococcygeus muscle even in Ca-free Krebs bicarbonate solution.(ABSTRACT TRUNCATED AT 250 WORDS)