Norepinephrine effects on SHR muscle vessel diameters, RBC and plasma flow

The distribution of blood flow through the cremaster microcirculation of pentobarbital anesthetized WKY and SHR rats was studied using fluorescence videomicroscopy and videodensitometry. The left cremaster muscle was spread over an optical port in a bath filled with modified krebs solution (pH = 7.4, 34 degrees C). The right femoral artery was cannulated with PE-10 tubing for the measurement of mean arterial blood pressure and bolus injections of indicators (DTAF-RBC's and FITC-dextran) into the aorta. The passage of indicators through the microvessels and vessel diameters were recorded on video tape during the topical application of norepinephrine (10(-10) M to 10(-6) M). Mean arterial blood pressure was 95 +/- 4 mmHg for WKY rats and 146 +/- 6 for SHR. Venular diameters were greater in WKY than in SHR. Norepinephrine decreased vessel diameters and blood flow velocity in a dose dependent manner. Mean transit time (t) for DTAF-RBC's was less than for FITC-dextran in first order arterioles for both normotensive and hypertensive animals. At high concentrations of norepinephrine differences in t for the two indicators disappeared. The t of both indicators were not different between normotensive and hypertensive animals in arterioles. However, t for both indicators were significantly increased in venules at all concentrations of norepinephrine in SHR but significantly less than in WKY. t for both indicators were increased in a dose dependent manner in both arterioles and venules. Arterioles of SHR appear to be constricted reducing the number of parallel circuits available for flow which allows passage of both RBC and plasma more directly and rapidly through shorter routes than in WKY, but restricts flow of RBC through the terminal arterioles.