Efficacy of sympathetic influences as related to initial vascular tonicity

The efficacy of vasoconstrictor influences as dependent on the initial vascular tone (initial rate of perfusion) and perfusion pressure was investigated in rat and frog hindlimb preparations perfused with saline solution. Efficacy was determined as the ratio of the preparation resistance (R) induced by sympathetic stimulation to the initial resistance R0. At a pressure normal for the experimental animal, the relationship between R/R0 and flow is expressed by a dome-shaped curve. At a pressure twice the normal, it is low at all points; at a pressure half the normal, different responses were obtained from the two species used in the experiment: in rats elevation of vascular tone induced by sympathetic stimulation resulted in a complete cessation of flow, whereas in frogs the initial high efficacy diminished with increasing vascular tone. A mathematical model devised to study the dependence of the efficacy of sympathetic stimulation on initial tonicity is described. The good consistency of experimental and calculated results is regarded as evidence that the prerequisites stipulated in the model, i.e., the capacity of closure and the heterogeneity of arterioles, in regard to the effort developed by their smooth muscle, are actually inherent in the vascular system of organs.