Role of prostaglandins and kinins in the renal pressor reflex

Intrarenal pelvic bradykinin-induced sympathoexcitatory reno-renal reflex is attenuated in rats exposed to chronic intermittent hypoxia

OBJECTIVE

In this study, we hypothesized that excitatory reno-renal reflex control of sympathetic outflow is enhanced in rats exposed to chronic intermittent hypoxia (CIH) with established hypertension.

METHODS

Under anaesthesia, renal sensory nerve endings in the renal pelvic wall were chemically activated using bradykinin (150, 400 and 700 μmol/l) and capsaicin (1.3 μmol/l), and cardiovascular parameters and renal sympathetic nerve activity (RSNA) were measured.

RESULTS

CIH-exposed rats were hypertensive with elevated basal heart rate and increased basal urine flow compared with sham. The intrarenal pelvic infusion of bradykinin was associated with contralateral increase in the RSNA and heart rate, without concomitant changes in blood pressure. This was associated with a drop in the glomerular filtration rate, which was significant during a 5 min period after termination of the infusion but without significant changes in urine flow and absolute sodium excretion. In response to intrarenal pelvic infusion of 700 μmol/l bradykinin, the increases in RSNA and heart rate were blunted in CIH-exposed rats compared with sham rats. Conversely, the intrarenal pelvic infusion of capsaicin evoked an equivalent sympathoexcitatory effect in CIH-exposed and sham rats. The blockade of bradykinin type 1 receptors (BK1R) suppressed the bradykinin-induced increase in RSNA by ∼33%, with a greater suppression obtained when bradykinin type 2 receptors (BK2R) and BK1R were contemporaneously blocked (∼66%).

CONCLUSION

Our findings reveal that the bradykinin-dependent excitatory reno-renal reflex does not contribute to CIH-induced sympathetic hyperactivity and hypertension. Rather, there is evidence that the excitatory reno-renal reflex is suppressed in CIH-exposed rats, which might relate to a downregulation of BK2R.

Factors influencing acute ischaemia-induced renal hypertension in rats

OBJECTIVE

To verify if the acute hypertension that occurs after reversal of complete renal ischaemia is related to the duration of ischaemia, is different in one-kidney (1K) and two-kidney (2K) rats, and is prevented by angiotensin receptor blockade.

METHODS

Four groups of Sprague-Dawley rats anaesthetized with pentobarbitone were studied before, during and after a reversible, complete renal ischaemia achieved by functional right nephrectomy.

RESULTS

In 1K rats (group 1, n = 21), reopening of right renal hilum after functional right nephrectomy of 180, 60 and 30 min was followed by peak increases in systolic blood pressure of 76.0 10.1 mmHg, 36.5 10.0 mmHg and 18.4 4.4 mmHg, respectively (mean SEM). In 2K rats (group 2, n = 21), functional right nephrectomy of 180, 60 and 30 min was followed by smaller increases in blood pressure of 49.8 7.6 mmHg, 5.9 3.3 mmHg and 8.3 2.1 mmHg, respectively. Plasma renin activity was directly related to the duration of functional right nephrectomy, and was greater in 1K rats. In group 3, irbesartan administered to 1K rats (n = 8) during functional right nephrectomy almost completely prevented the development of hypertension upon reopening. In group 4, labetalol injected intravenously in 1K rats (n = 3) did not prevent the blood pressure surge at reopening (49.2 8.5 mmHg).

CONCLUSIONS

An experimental acute renal hypertension may be elicited both in 1K and in 2K rats and for functional right nephrectomy of 30, 60 and 180 min duration. The increase in blood pressure is proportional to the duration of functional right nephrectomy and greater in 1K than in 2K rats. The experimental acute renal hypertension is due to acute release of renin and generation of endogenous angiotensin II, and is specifically prevented by the angiotensin II type 1 receptor blocker, irbesartan, but not by labetalol.

Influence of sympathetic blockade on the acute hypertensive response to aortic constriction

The objective of the present study was to determine the contribution of the sympathetic nervous system to the hypertensive response to acute (45-min) aortic coarctation in conscious intact or sinoaortic-denervated (SAD) rats. Rats were treated chronically (5 wk) with guanethidine (50 mg.kg-1.day-1 i.p.) to induce sympathetic nerve degeneration or acutely with the alpha 1-adrenergic receptor antagonist prazosin. (1 mg/kg i.v.). Aortic constriction elicited a prompt and sustained rise in mean carotid pressure that was significantly greater in SAD than in intact rats. The increase in pressure was associated with reflex bradycardia only in the intact rats, whereas the heart rate of SAD rats did not change. Guanethidine treatment did not affect the arterial pressure or heart rate responses to aortic coarctation of intact rats but blunted the hypertensive response of SAD rats to the same values exhibited by intact rats. Prazosin administered 10 min after the beginning of aortic coarctation reduced the hypertensive response of SAD rats to the same level as that of intact rats. In conclusion, the data obtained by means of the association of sinoaortic deafferentation with chronic sympathectomy with guanethidine or acute alpha 1-adrenergic receptor blockade with prazosin indicate that the greater hypertensive response of SAD rats involves a lack of suppression of the sympathetic activity in the maintenance of the rise in pressure elicited by aortic coarctation.

Neural control of renal function

The renal nerves are the communication link between the central nervous system and the kidney. In response to multiple peripheral and central inputs, efferent renal sympathetic nerve activity is altered so as to convey information to the major structural and functional components of the kidney, the vessels, glomeruli, and tubules, each of which is innervated. At the level of each of these individual components, information transfer occurs via interaction of the neurotransmitter released at the sympathetic nerve terminal-neuroeffector junction with specific postjunctional receptors coupled to defined intracellular signaling and effector systems. In response to normal physiological stimuli, changes in efferent renal sympathetic nerve activity contribute importantly to homeostatic regulation of renal blood flow, glomerular filtration rate, renal tubular epithelial cell solute and water transport, and hormonal release. Afferent input from sensory receptors located in the kidney participates in this reflex control system via renorenal reflexes that enable total renal function to be self-regulated and balanced between the two kidneys. In pathophysiological conditions, abnormal regulation of efferent renal sympathetic nerve activity contributes significantly to the associated abnormalities of renal function which, in turn, are of importance in the pathogenesis of the disease.

Mechanisms of renal vasoconstriction following furosemide in conscious rats

Regional hemodynamics following intravenous injection of furosemide were studied in conscious rats instrumented with Doppler flowprobes. Furosemide caused a dose-dependent acute (within 3 min) transient increase in renal resistance (RR) followed by a later generalized vasoconstriction in the renal, mesenteric and hindquarter (HQR) vascular bed. With in vitro experiments a possible direct effect of furosemide on renal artery segments was excluded. The acute rise in RR was partly (approximately 40%) attenuated by pretreatment with phentolamine or prazosin, but not prevented by renal denervation, volume repletion or pretreatment with captopril, losartan, indomethacin, aminophylline, nifedipine, or a vasopressin V1A-receptor-antagonist. However, the acute renal vasoconstriction was absent in water diuretic rats. The later vasoconstriction was blunted by captopril (RR+HQR), losartan (RR) and phentolamine (HQR) pretreatment. We conclude that the later generalized vasoconstriction following furosemide involves regional specific stimulation of angiotensin II and alpha-adrenoceptors. The mechanism of the acute renal vasoconstriction could not be fully determined. Because of its absence in water diuretic rats, its rapid onset, transient nature and relative insensitivity for pharmacological tools in euvolemic rats, the acute increase in RR following furosemide may be caused by cell swelling as a result from dissipation of the renal medullary gradient.