On the characterization of dopamine-induced contractile response of rat anococcygeus muscle preparation

Presence of specific dopamine (DA) receptors and their characterization was attempted in rat anococcygeus muscle preparation. Dopamine (10 -6 M) and B-HT 920 (10 -6 M) produced concentration dependent contractions of the rat anococcygeus muscle preparation. The response of DA was shifted towards right in presence of haloperidol (10 -6 M; pA 2 = 6.8) and sulpiride (10 -4 M) in a competitive manner. α 2 antagonists yohimbine (10 -5 M) and idazoxan (10 -5 M) blocked the response to DA in a competitive manner, while α 1 antagonist prazosin (10 -5 M) completely blocked the response to DA. SCH 23390 (10 -5 M), a D 1 DA antagonist potentiated the response to DA. Reserpinization (5 mg/kg, 24 hr prior) brought about a shift towards the right, and this response was similarly blocked by haloperidol, sulpiride and yohimbine without affecting the maximum response. Desipramine (10 -5 M) blocked the response of DA in a non-competitive manner. Pretreatment of animals with desipramine (10 mg/kg) followed by reserpine, brought about a reversal of action of reserpine. The response to B-HT 920 (10 -6 M), was blocked similarly by haloperidol and yohimbine. However, the effect of desipramine was more pronounced when compared to DA per se. SKF 38393, a D 1 DA agonist, potentiated the response to B-HT 920. The findings suggest the presence of both D 1 and D 2 DA receptors in rat anococcygeus muscle and that DA also acts on adrenergic receptors to produce a contractile response of this muscle preparation.