Effect of nephrectomy and a low sodium diet on the increase in plasma renin levels produced by hemorrhage

Differential regulation of renal prostaglandin receptor mRNAs by dietary salt intake in the rat

BACKGROUND

In this study, we tested the hypothesis that prostaglandin (PG) receptor expression in the rat kidney is subject to physiological regulation by dietary salt intake.

METHODS

Rats were fed diets with 0.02 or 4% NaCl for two weeks. PG receptor expression was assayed in kidney regions and cells by ribonuclease protection assay and reverse transcription-polymerase chain reaction analysis. Functional correlates were studied by measurement of PGE2-induced cAMP formation and renin secretion in juxtaglomerular (JG) cells isolated from animals on various salt intakes.

RESULTS

EP1 and EP3 receptors were predominantly expressed, and the EP2 receptor was exclusively expressed in the rat kidney medulla. The EP4 receptor was strongly expressed in glomeruli and in renin-secreting JG granular cells. IP receptor transcripts were found mainly in cortex. Maintaining rats on a low- or high-NaCl diet did not affect the expression of EP1 or IP receptors, whereas EP4 transcripts in glomeruli were increased twofold by salt deprivation. Consistent with this, we found that PGE2-evoked cAMP production and renin secretion by JG cells from salt-deprived animals were significantly higher compared with cells obtained from salt-loaded animals. In the outer medulla, EP3 transcripts correlated directly with salt intake, and mRNA abundance was increased twofold by a high-NaCl diet.

CONCLUSIONS

Our results suggest that subtype-specific, regional changes in PG receptor expression are involved in the renal adaptation to changes in salt intake. The results are in accord with the general concept that renocortical PGE2 stimulates renin secretion and maintains renal blood flow during low-salt states, whereas medullary PGE2 promotes salt excretion in response to a high salt intake.

Effect of bilateral nephrectomy on the recovery of blood pressure after acute hemorrhage in rats: role of renin-angiotensin system

The effect of bilateral nephrectomy, and administration of an inhibitor of angiotensin converting enzyme, on the recovery of arterial blood pressure after hemorrhage (loss of 1% of b.wt), was studied in male Sprague-Dawley rats. Neither manoeuver significantly affected the recovery of blood pressure within the first 10 min after hemorrhage. Thereafter, the recovery of the blood pressure was markedly suppressed. The study suggests that the initial recovery of blood pressure is unrelated to the kidneys, but the later one requires their presence and depends on the activity of the renin-angiotensin system.

A steady-state control analysis of the renin-angiotensin-aldosterone system

This communication develops a steady-state block diagram about the configuration of the renin-angiotensin-aldosterone system in relation to the mineralocorticoid-dependent reabsorption of sodium. Two principal mechanisms appear to be involved in the control of renin secretion. The baroreceptor hypothesis suggests that decreased stretch of the afferent glomerular arteriole produces increased renin secretion. The macula densa theory, on the other hand, suggests that renin secretion responds to alterations either in sodium load or in sodium concentration at the macula densa. These two receptors are presented as parallel feedback pathways which are independently capable of altering renin secretion. This concept of independent receptor function is supported by literature in which one of the receptor pathways was eliminated. The system configuration for the renin-angiotensin-aldosterone system suggests the hypothesis that the vascular receptor controls renin secretion in the autoregulatory range of blood pressure and that macula densa control dominates at pressures below the autoregulatory range. The control system diagram also calls attention to certain unique features of the renin-angiotensin-aldosterone system; it indicates that (1) a chronic low-sodium diet involves changes in both input variables (extracellular fluid volume) and parameters (hepatic blood flow) to maximize sodium reabsorption, (2) thoracic inferior vena cava constriction opens the feedback loop so that the renal receptors do not perceive increases in extracellular fluid volume, (3) congestive heart failure reduces the overall gain of the system, and (4) a small nonhypotensive hemorrhage primarily increases renin secretion through renal sympathetic nerve stimulation.

Renin release after hemorrhage and after suprarenal aortic constriction in dogs without sodium delivery to the macula densa

To study the possible factors that control renin secretion by the kidney, a model has been developed to prevent glomerular filtration in the dog. Ureteral ligation was combined with a 2-hour period of total renal ischemia to induce tubular degeneration and cessation of glomerular filtration. After the surgical procedures, the dogs were maintained for 4 days by peritoneal dialysis. The absence of lissamine green dye in the renal tubules after an intra-aortic injection provided evidence that filtration had ceased. Plasma renin activity was significantly increased in six conscious dogs at 30, 60, and 90 minutes after hemorrhage of 20 ml/kg body weight. In another group of five conscious animals, suprarenal aortic constriction produced increases in plasma renin activity at 30, 60, and 90 minutes that were 3 to 4 times the control values before aortic constriction. Plasma renin substrate concentration did not change significantly after hemorrhage or after aortic constriction. In a third group of five dogs, only the left kidney was subjected to ureteral ligation and renal ischemia. These dogs were anesthetized and renal blood flow was measured with an electromagnetic flowmeter. After hemorrhage (20 ml/kg), renin secretion increased significantly above control levels at 30, 45, and 60 minutes. It is concluded that sodium delivery to the macula densa is not essential for renin secretion.