Effects of Transmural Distending Pressure on Integrated Venous Function in Normal Rat.
Vasomotor tone is largely maintained by sympathetic nerves, myogenic reactivity and key local and circulating hormones. Acting together, these factors ensure moment-to-moment adjustments of net vascular tone required to maintain hemodynamic stability. In rat mesenteric small veins (MSV) and arteries (MSA), we investigated the contribution of the endothelium, L-type voltage operated calcium channels (L-VOCCs), PKC and Rho kinase to myogenic reactivity. The interaction of myogenic reactivity with norepinephrine (NE), endothelin-1 (ET-1), and sympathetic nerve activation was also investigated under conditions of changing transmural distending pressure. We also evaluated the relative contribution of alpha adrenergic (α-A) and endothelinergic receptors to NE and ET-1 contractile responses, respectively. Additionally, the effects of changing transmural pressure on endothelial dilator function of MSV were examined. Myogenic reactivity was not altered by nitric oxide synthase (NOS) inhibition or endothelium removal in both vessels. L-VOCCs blockade completely abolished arterial tone, while only partially reducing venous tone. PKC and Rho kinase inhibitors largely abolished venous and arterial myogenic reactivity. Increasing transmural pressure did not alter NE, ET-1, and bradykinin responses, but it significantly reduced neurogenic contractions. MSV were more sensitive to NE, ET-1 and sympathetic nerve activation compared with corresponding arteries. α-A and ET-1 receptor agonist and antagonist application revealed the participation of α1-A and ETA receptors in NE and ET-1 contractile responses, respectively. α2-A and ETB receptors appeared to mediate NE and ET-1 responses in MSV, respectively. Bradykinin induced-vasodilation was mainly reduced by NOS inhibition, and BKCa and SkCa blockade. These results suggest that myogenic factors are important contributors to net venous tone in MSV; PKC and Rho kinase activation are important to myogenic reactivity in both vessels, while L-VOCCs play a limited role in the veins versus the arteries; mesenteric veins maintain an enhanced sensitivity to NE, ET-1 and sympathetic nerve activation compared to the arteries with neurogenic contractions being affected by transmural pressure elevations; α1-ARs and ETA are the predominant receptors mediating contractile responses to NE and ET-1, respectively, with functional evidence indicating the presence of α2-ARs and ETB receptors in MSV; and venous endothelial dilator function is not affected by an elevation of transmural pressure.