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Contractile responses to noradrenaline: varying dependence on external calcium of consecutive vascular segments of perfused rat hindquarters.

To study the dependency of vascular smooth muscle contractions, produced by noradrenaline, on the concentration of extracellular calcium, experiments were performed on adult male Wistar rats. The hemodynamic characteristics of the isolated, perfused hindquarters were investigated from maximal dilatation up to maximal, noradrenaline-induced constriction under constant flow conditions. Pressor responses to noradrenaline in the consecutive segments of the vascular bed ("proximal" and "distal" precapillary resistance vessels and postcapillary resistance vessels) were determined at low (0.2mM) and at normal (1.5-2.0 mM) Ca++ concentrations in the perfusate. Dependence on external calcium is much greater peripherally. The smallest pre- and postcapillary resistance vessels are the most dependent while the larger, "proximal" resistance vessels are the least dependent on the availability of external calcium. The results illustrate the considerable differentiation of smooth muscle, depending on its location along the vascular circuit. They further indicate that it is likely that the hemodynamically so important microvessels are highly dependent on extrinsic Ca++ sources not only concerning their involvement of remote adrenergic control, but also in their maintenance of normally pronounced "myogenic" tone.