The divergence in the excitation-contraction coupling of rat mesenteric resistance arteries lies distal to the receptor site.

In vitro studies were performed with rat mesenteric resistance arteries (diameter 158-271 microns) mounted as ring preparations in an isometric myograph. Noradrenaline evoked concentration-dependent, tonic contractions in calcium-containing media, whereas transient responses were elicited by the agonist in a concentration-dependent manner in calcium-free media; the pD2 values (-log EC50) were 6.73 and 5.71, respectively. The responses to noradrenaline in calcium-containing media were competitively inhibited by the alpha 1-selective antagonist, prazosin (pA2:9.23), and the responses to noradrenaline in calcium-free media were abolished by 10 nM prazosin. The agonist affinities expressed as the pKD values (-log KD) for the receptor mediating the response to noradrenaline in calcium-containing and calcium-free media were 5.78 and 5.64, respectively. These data suggest that although much higher concentrations of noradrenaline are required to elicit responses in calcium-free media than in calcium-containing media, both types of response are mediated by one adrenoceptor subtype, the alpha 1-adrenoceptor.