Localization and Effects of Calcitonin Gene‐Related Peptide in the Testicular Vasculature of the Rat

UNLABELLED: Using laser Doppler flowmetry, the effects of unilateral intratesticular injection of calcitonin gene-related peptide (CGRP) and CGRP8-37, a CGRP-receptor antagonist, on right- and left-testicular blood flow and mean arterial pressure were studied on anesthetized adult rats. Calcitonin gene-related peptide in doses of 5 and 50 ng increased blood flow 37 +/- 11% (mean +/- SEM, P < 0.05) and 30 +/- 5% at 5 mm, but not 15 mm, away from the injection site, respectively. They did not influence mean arterial pressure nor blood flow in the contralateral testis. Five-hundred nanogram doses increased testicular blood flow in the injected testis at a point 15 mm away from the injection site (22 +/- 3%, P < 0.05) and caused a slight decrease in mean arterial pressure (-12 +/- 3%, P < 0.05). The highest dose, 5 micrograms, caused a large (-39 +/- 3%, P < 0.05) fall in mean arterial pressure within 1 minute after injection, and testicular blood flow was reduced in both the injected (-9 +/- 2%, P < 0.05, 15 mm away from injection site) and contralateral testis (-20 +/- 5%, P < 0.05). Pretreatment with 500 ng of the receptor antagonist, CGRP8-37, did not significantly attenuate the blood flow increasing affect of 50 ng CGRP, nor did 50 micrograms CGRP 8-37 (given alone) influence basal testicular blood flow in the injected testis. Using Immunohistochemistry, CGRP-containing nerves were observed in the superior and interior spermatic nerves, in the testicular artery, and in the veins leaving the testis but not in intratesticular blood vessels. CONCLUSIONS: 1) CGRP is a potent vasodilator in the testicular vasculature and it may be involved in the local regulation of testicular blood flow: 2) the testis has limited capacity to autoregulate and is consequently unable to maintain a constant testicular blood flow during large and rapid reductions in blood pressure, and 3) the local and systemic effects of vasodilators act in opposite directions in the testis.