Beta-Adrenergic Mechanisms in Arterial Hemodynamics: A Comparison between Normotensive and Hypertensive Rats

The purpose of this study was to determine whether beta-adrenergically mediated cardiovascular functions such as arterial pressure (AP), heart rate (HR), stroke volume (SV), cardiac output (CO), peripheral resistance (R(p)), arterial impedance (Z(c)), mean arterial compliance (C(m)) and pulse wave reflection (P(b)) were altered in the spontaneously hypertensive rat (SHR) compared to the normotensive Wistar Kyoto rat (WKY). In pentobarbital-anesthetized and artificially ventilated rats, the aortic pressure wave was recorded with a high-fidelity Millar sensor, and aortic flow wave with an electromagnetic flow probe. The pressure and flow waves were subjected to Fourier transform so as to analyze impedance spectra. Acute beta-adrenergic blockade was produced by an intravenous injection of propranolol (nonselective) and atenolol (selective beta(1)-blocker) at doses of 2 and 5 mg/kg, respectively. Steady-state parameters were obtained 15-20 min after intravenous administration. The SHR had higher AP, HR, R(p) and Z(c) than the WKY. SV and CO remained unaltered while C(m) was lower. In response to propranolol, the mean AP was increased by 7 mm Hg in the WKY, but did not change in the SHR. Moreover, significant decreases occurred in HR, CO and C(m) in addition to increases in R(p), Z(c) and P(b). These changes between the SHR and WKY were only slight. Atenolol caused decreases in AP, HR and CO in both SHR and WKY, but did not significantly alter the R(p), Z(c), C(m) and P(b). Again, the atenolol-induced changes in AP, HR and CO did not appear to be significantly different between SHR and WKY. The results indicate that beta-adrenergic effects on the heart, Windkessel and resistance vessels are neither greatly enhanced nor impaired during the development of hypertension. In the hypertensive state, significant beta-adrenergic mechanisms still exert tonic vasodilatory effects on the large and small arterial system. Copyright 1996 S. Karger AG, Basel