Sensitivity of renin secretion to volume depletion in the anaesthetized dog: comparison between urinary drainage and slow haemorrhage

Effects of renal denervation and atrial natriuretic factor on tubular reabsorption in anaesthetized rats

1. The effects of acute unilateral renal denervation were examined in 17 anaesthetized rats. Renal haemodynamic changes were monitored using standard clearance techniques. Lithium clearance was used to assess fractional proximal sodium and water reabsorption. 2. Denervation resulted in ipsilateral renal vasodilatation with marked natriuresis and diuresis, a small increase (15%, P less than 0.05) in glomerular filtration rate (GFR) and a consequent reduction in filtration fraction. Fractional lithium reabsorption decreased (67.3 +/- 2.9% to 54.5 +/- 4.0%, P less than 0.01) and absolute proximal reabsorption did not change, indicating impairment of proximal glomerulotubular balance (GTB). No similar changes in haemodynamic or transport parameters were observed in the contralateral, innervated kidney, although vascular resistance increased. 3. In 9 experiments following denervation of the left kidney, systemic low dose infusion (10 ng/min) of atrial natriuretic factor (ANF) resulted in a fall in mean arterial blood pressure from 116 +/- 3 mmHg to 107 +/- 3 mmHg (P less than 0.05). In the denervated kidney ANF increased urine flow rate and sodium excretion to rates above those established following denervation alone. However, in the right kidney, despite the increased filtered load (35%, P less than 0.01), the natriuretic and diuretic responses to ANF were abolished. 4. In the denervated kidney, ANF further reduced the fractional reabsorption of lithium from 53.6 +/- 2.3% to 45.6 +/- 3.8% (P less than 0.05). GFR increased by 32% (a total of 49% higher than during pretreatment) but absolute proximal reabsorption (APR) did not change. However, in the right, innervated kidney ANF infusion produced a 35% increase in GFR accompanied by a 53% rise in APR. 5. It is concluded that the natriuresis induced by unilateral denervation is due predominantly to impaired proximal GTB. The natriuretic action of ANF was associated with further impairment of proximal GTB, not dependent upon decreasing activity of renal sympathetic nerves, but was abolished when filtration fraction and renal sympathetic tone were elevated.

Ouabain inhibits renin release by a direct renal haemodynamic effect

The effect of intrarenal infusion of ouabain (90 micrograms/kg) on renin release was examined in the anaesthetized dog. Ouabain reduced cortical Na-K-ATPase activity to 23% and outer medullary activity to 18% of the control level. During renal arterial constriction to a perfusion pressure below the autoregulatory range, renin release rose from 1.2 +/- 0.4 to 47.4 +/- 6.9 micrograms/min (P less than 0.001). This response was abolished by ouabain. When superimposed on renal arterial constriction, beta-adrenergic stimulation enhanced renin release from 25.6 +/- 10.7 to 56.9 +/- 9.5 micrograms/min (P = 0.02) at a urinary sodium excretion of 2 +/- 1 mumol/min. After ouabain, the corresponding increment substantially decreased since release rose from 5.6 +/- 2.0 to 19.9 +/- 5.3 micrograms/min only (P = 0.02), at a urinary sodium excretion of 140 +/- 67 mumol/min. When glomerular filtration was reduced to zero by ureteral occlusion in one series, renin release increased to 22.6 +/- 5.1 but was reduced (P less than 0.05) by ouabain to 13.5 +/- 5.5 micrograms/min and superimposed isoproterenol had no effect. According to these observations, ouabain inhibits renin release by a direct effect on the afferent arteriole through constriction of the autoregulating renin-secreting segment.

Influence of renal nerves on renin secretion in the conscious dog

In order to evaluate the influence of renal nerves on renin secretion during changes in blood volume, we studied the mean arterial blood pressure (MAP) and the renal venous plasma renin activity (PRA) in 6 conscious dogs having one intact and one denervated kidney. After a passive head-up tilt PRA increased significantly in the vein of the intact kidney while it remained stable in the denervated one. The intravenous injection of Furosemide (3 mg/kg) induced a rapid elevation of PRA in both renal veins. The kinetics of the variations of renin secretion were similar in the two kidneys, but its magnitude was significantly lower in the denervated side. After a slow hemorrhage of 2, 4 and 6 ml/kg, MAP was unchanged and PRA increased in both renal veins but in a significantly lower degree in the denervated side. When blood loss reached 8 ml/kg, MAP decreased and PRA increased identically in the two renal veins. It was concluded that, in conscious dogs, the renal nerves could participate in the rapid adaptations of renin secretion during small changes in the circulating blood volume.