The modulation of ureteral smooth muscle contractile responses by α 1 - and α 2 -adrenoceptor activation.

PURPOSE: This study was performed to investigate the influence of α-adrenoceptor subtypes upon ureteral smooth muscle contractile responses.

METHODS: Rat ureters were challenged in vitro with noradrenaline (NA), the α1 -adrenoceptor agonist phenylephrine (PE), and the α2 -adrenoceptor agonist clonidine (CLON). The influences of the agonists on the magnitude and frequency of acetylcholine (ACh)-stimulated phasic contractile responses were recorded.

RESULTS: The magnitude of the phasic contractile responses effected by ACh was not significantly influenced by the adrenoceptor agonists, but the frequency of the response was significantly enhanced by all three agonists (p < 0.05). Idazoxan and prazosin abolished the rise in frequency effected by CLON and PE, respectively, whereas both antagonists in combination were required to abolish the increase in frequency effected by NA.

CONCLUSIONS: It has been demonstrated that α1 - and α2 -adrenoceptors modulate the contractile function of rat ureteral smooth muscle by increasing the frequency, but not the magnitude, of phasic contractile responses. The enhancement of contractile function by NA is mediated by mechanisms dependent upon both α1 - and α2 -adrenoceptors.