Mechanisms leading to increased vasodilator responses to calcitonin-gene-related peptide in mesenteric resistance arteries of early pregnant rats.

The objective of this study was to explore the mechanism responsible for the higher relaxing responses of mesenteric arteries to calcitonin-gene-related peptide (CGRP) in pregnancy. We performed myograph and ligand binding studies to determine the role of matrix metalloproteinase-2 (MMP-2) and CGRP receptor density. MMP activity was manipulated in isolated arteries by exposing them to the blocking effects of doxycycline. Vascular activity of MMP-2 was studied by gelatin zymography, and CGRP receptor density was determined by ligand binding analysis. Compared to nonpregnant rats, CGRP elicited stronger arterial relaxation in pregnant rats. The latter effect was neither accompanied by a change in relaxing responses to direct activation of adenylyl cyclase by forskolin nor by a change in the response to stimulation of G-protein-coupled adrenergic receptors by isoproterenol. Doxycycline did not affect the stronger arterial relaxation in pregnancy in spite of the observed more than threefold higher arterial MMP-2 activity. Density of binding sites for [(125)I]CGRP in arteries from pregnant rats (64 +/- 14 fmol/mg protein) and from virgin rats (54 +/- 5 fmol/mg protein) were comparable. The results of this study provide evidence for increased coupling of CGRP receptors to adenylyl cyclase in early pregnancy.