Intact hindquarter vascular responses of young spontaneously hypertensive rats to norepinephrine and tyramine

Abstract

Intact hindquarter vascular responses to abdominal aortic injections of subpressor doses of norepinephrine (0.01, 0.02, 0.03 μg) or tyramine (5, 10, 15 μg) were examined in young ( 2 1 2 –3 months ) spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) normotensives to ascertain whether altered vascular response to catecholamines in SHR could be detected in the presence of relatively constant systemic arterial perfusion pressure. Increases in vascular resistance (Δ mmHg. min/ml) and total decreases in blood flow volume (Δ ml) were determined by using electromagnetic flowmetry and blood flow integration techniques. Under a resting condition the abdominal aortic flow rate (ml/min) was similar between the SHR (8.7 ± 0.5) and WKY control (9.1 ± 0.5), whereas hindquarter vascular resistance was greater (73.8%) in SHR than in WKY normotensives (P < 0.05). The increase in vascular resistance in response to a low dose of norepinephrine (0.1 μg) was greater (85%) in SHR than in WKY rats (P < 0.05) and at higher doses of norepinephrine (0.02, 0.03 μg) there was a tendency of greater increase in resistance (20–30%) in SHR (0.05 < P < 0.1). Tyramine at all doses tested produced greater increases (50–66%) in resistance in SHR compared to WKY normotensives (P < 0.05). On the other hand, the decreases in the integrated total blood flow volume passing to the hindquarters after norepinephrine or tyramine administration at all doses were less (27–46%) in SHR than in WKY control (P < 0.05). The data demonstrate increased catecholamine vasoconstrictor responses in the intact hindquarters of SHR with attenuated blood flow volume decreases due to the higher resting vascular resistance, supporting the contention that the elevated vascular resistance in SHR may be attributed to vasoconstrictor hyperresponsiveness of catecholamines.