Selective variation of agonist and neurally mediated vasoconstriction with rabbit middle cerebral artery branch order

Abstract

The in vitro contractions of four successive branches of the rabbit middle cerebral artery to norepinephrine (NE), serotonin (5HT), histamine, and electrical field stimulation (EFS) were examined. Internal lumen diameters of the four branches (designated MCA, M1, M2, M3, proximal to distal) ranged from a mean of 270 microns in the MCA to 160 microns in the M3 segments. NE produced a small alpha adrenoceptor-mediated response in the MCA (12% of tissue maximum); this was diminished in the M1 segment and there was no evidence of alpha adrenoceptor-mediated responses in the M2 and M3 branches. NE contractions, mediated through low affinity sites (extraceptors), did not change with branch order. Maximal responses to 5HT (1 microM) were small and also diminished with branch order; the MCA and M1 contracted to 24 and 18% of tissue maximum, respectively. Negligible responses were obtained in M2 and M3. EFS resulted in contractions that were tetrodotoxin- and guanethidine-sensitive. Maximum EFS responses were 21% tissue maximum in the MCA, 13% in the M1, and less than 5% in the M2 and M3. In contrast, maximal contractile responses to histamine (100 microM) were at or near tissue maximum in all segments. Removal of the endothelium did not influence the size of the contraction to NE, 5HT or histamine in the MCA and M1 segments. Neither NE, 5HT, nor histamine caused relaxation responses in agonist or KCl-contracted segments. The results indicate a differential variation in responsiveness with increasing branch order in the rabbit cerebrovascular bed.(ABSTRACT TRUNCATED AT 250 WORDS)