Circulating and tissue catecholamines in rats with chronic neurogenic hypertension

Arterial baroreflex control of renal hemodynamics in humans

BACKGROUND

Control of renal hemodynamics by the arterial baroreflex has never been proved in humans. Apart from the physiological viewpoint, this issue is relevant because altered baroreflex function has been implicated in the pathogenesis of human hypertension.

METHODS AND RESULTS

Renal function studies were performed in seated healthy volunteers (n = 12; age range, 20 to 34 years) during sustained neck suction at -60 mm Hg, aiming to selectively activate the carotid sinus arterial baroreceptors. Two protocols were followed. One group of 6 volunteers taking a 20 mmol/d sodium diet underwent 90 minutes of neck suction. Compared with a time-control study, neck suction decreased arterial pressure and heart rate; increased glomerular filtration rate (inulin clearance) from 104 +/- 6 to 114 +/- 8 mL/min (P < .01), renal plasma flow (para-aminohippurate clearance) from 616 +/- 52 to 665 +/- 42 mL/min (P < .01), and renal blood flow (from 1120 +/- 95 to 1209 +/- 77 mL/min, P < .01); and decreased renal vascular resistance (from 86 +/- 8 to 76 +/- 6 mm Hg.min.L-1, P < .01). Neck suction had no effect on plasma renin activity, aldosterone, atrial natriuretic peptide, catecholamines, and renal sodium excretion. The other 6 volunteers took a normal sodium diet and underwent sustained neck suction for 60 minutes. In this group, no effects on renal hemodynamics could be discerned, despite a modest decrease in blood pressure and heart rate.

CONCLUSIONS

These data show, for the first time, that the arterial baroreflex is involved in the control of renal hemodynamics in humans. However, basal arterial baroreflex control of renal hemodynamics is probably low, and arterial baroreflex activation with subsequent renal vasorelaxation may be found only in conditions in which basal arterial baroreflex control of kidney function is significant, as is presumably the case in seated sodium-restricted subjects.

Changes in cardiovascular adrenoceptor response in rats subsequent to myocardial infarction

Since sympatho-adrenal activity was greatest on the second day after experimental myocardial infarction in the rat, we investigated the responsiveness of cardiovascular adrenoceptors in pithed rats at this time and also determined the number of beta-adrenoceptors in the myocardium of the rats. In addition, the sympathetic outflow was measured, as a presynaptic parameter for estimating sympathetic nervous activity. The number of beta-adrenoceptor binding sites and the frequency responses of the heart were unchanged by myocardial infarction on the second day. There was also no difference from control animals with respect to sympathetic outflow. However, the rise in diastolic blood pressure, caused by electrical stimulation of the sympathetic nervous system, was far more pronounced in animals with infarction than in controls. In contrast, rats with myocardial infarction exhibited an attenuated response of diastolic blood pressure to infused noradrenaline. The observed effects can possibly be explained by changes of alpha-adrenoceptors, or perhaps by an enhanced degradation of catecholamines in the endothelium.