Thomas Maack

Effects of auriculin (atrial natriuretic factor) on blood pressure, renal function, and the renin-aldosterone system in dogs

Auriculin is a potent vasoactive and natriuretic peptide that was recently isolated and purified from rat atrial tissue. Since this peptide could be of great importance for renal, cardiovascular, and volume homeostasis, its functional properties have been characterized in dogs. The effects of synthetic auriculin on renal function, mean blood pressure, plasma renin activity, renin secretory rate, and plasma aldosterone levels were determined. Auriculin was administered intravenously as a prime (1.0 microgram/kg body weight) and constant infusion (0.1 microgram per minute/kg body weight for one hour) to five anesthetized dogs. In addition, two conscious dogs were used to verify some of the results obtained in anesthetized dogs. Auriculin decreased mean blood pressure from 134 +/- 5 to 122 +/- 4 mm Hg (p less than 0.05, paired t test) and increased glomerular filtration rate (25.5 +/- 2.7 to 32.4 +/- 4.1 ml per minute per kidney, p less than 0.05), diuresis (0.21 +/- 0.03 to 1.06 +/- 0.14 ml per minute per kidney, p less than 0.05), natriuresis (38 +/- 0.6 to 187 +/- 35 mueq per minute per kidney, p less than 0.05), and kaliuresis (14.8 +/- 1.6 to 35.7 +/- 6.3 mueq per minute per kidney, p less than 0.05). These effects were sustained throughout the infusion of auriculin and were entirely reversible. Renal plasma flow increased transiently for one to two minutes, and then returned to or below control levels. Urine osmolality decreased by 40 percent (p less than 0.05) whereas free water clearance remained unchanged (p less than 0.05). Auriculin reversibly decreased plasma renin activity (11.6 +/- 2.3 to 3.6 +/- 1.2 ng/ml per hour, p less than 0.05), renin secretory rate (895 +/- 313 to 255 +/- 28 ng per hour per minute, p less than 0.05), and plasma aldosterone levels (8.4 +/- 1.6 to 3.6 +/- 0.7 ng/dl, p less than 0.05), whereas plasma cortisol levels remained unchanged. These results demonstrate that auriculin has a unique combination of functional properties, increasing glomerular filtration rate, diuresis, and natriuresis, without a sustained increase in total renal blood flow, and lowering blood pressure, plasma renin levels, renin secretory rate, and plasma aldosterone levels. These properties suggest an important potential role for atrial natriuretic peptides in the regulation of renal function, extracellular volume, and blood pressure.

Atrial natriuretic factor increases hematocrit and decreases plasma volume in nephrectomized rats.

To test the hypothesis that ANF increases the efflux of fluid from capillaries we determined the effect of synthetic (1-28) ANF on hematocrit (Hct) and plasma volume (PV) in 24 hr bilaterally nephrectomized rats. Results were compared with those obtained in bilaterally nephrectomized rats receiving saline alone (S) or an infusion of sodium nitroprusside (NP) to produce a similar blood pressure lowering effect as ANF. PV was determined by the volume of distribution of RISA 10 minutes after its i.v. administration. After control periods (C) rats were infused for 40 minutes with saline alone (group A, n = 8), (1-28)ANF, 0.3 micrograms X min-1/kg body wt (group B, n = 9), 1.0 microgram X min-1/kg body wt (group C, n = 8) and NP, 1.0 to 2.0 micrograms X min-1/kg body wt (group D, n = 8). Recovery periods (R) were performed 45 minutes after the experimental periods (E). During E periods, mean arterial blood pressure did not change in group A and fell significantly (p less than 0.01) in the other groups. Group B: C = 89 +/- 4; E = 82 +/- 4 mmHg. Group C: C = 106 +/- 7; E = 92 +/- 7 mmHg. Group D: C = 104 +/- 5; E = 90 +/- 5 mmHg. Neither saline or NP changed Hct, whereas, ANF significantly (p less than 0.01) increased this parameter (Group B: C = 43.4 +/- 0.7; E = 46.3 +/- 0.6% and Group C: C = 44.5 +/- 1.2; E = 47.4 +/- 1.2%). ANF significantly decreased PV (3.86 +/- 0.14 ml/100 gm body wt) as compared to S (4.51 +/- 0.22 ml/100 gm body wt; p less than 0.05) or NP (4.69 +/- 0.11 ml/100 gm body wt; p less than 0.01). The average decrease in PV of 14% corresponded to the average increase in Hct of 7%. Results demonstrate that ANF increases the efflux of fluid from capillaries. This effect explains, at least in part, the ANF-induced decrease in cardiac output and blood pressure in normotensive animals and may play an important role in the homeostatic regulation of plasma volume.

Cardiovascular effects of atrial natriuretic factor in anesthetized and conscious dogs.

Atrial natriuretic factor lowers blood pressure in normotensive and hypertensive animal models. The present study examined the mechanism of the blood pressure-lowering effect in 10 normotensive dogs. Four awake dogs previously instrumented with electromagnetic flow probes for measurement of cardiac output and catheters for systemic hemodynamic and cardiac dynamic measurements were studied. After a 30-minute control period, a 3 micrograms/kg bolus followed by 0.3 micrograms/min/kg of a 24-residue synthetic atrial natriuretic factor was infused for 30 minutes, followed by a 1-hour recovery period. Mean arterial pressure fell significantly during infusion (control, 125 +/- 4; infusion, 108 +/- 5; recovery, 125 +/- 9 mm Hg; p less than 0.05) and was accompanied by a slight but significant bradycardia (control, 144 +/- 7; infusion, 134 +/- 5; recovery, 145 +/- 7 beats/min; p less than 0.05). Significant reductions in cardiac output (control, 2.66 +/- 0.60; infusion, 2.18 +/- 0.60; recovery, 2.74 +/- 0.60 L/min; p less than 0.05), stroke volume (control, 18.4 +/- 3.9; infusion, 16.0 +/- 4.2; recovery, 19.0 +/- 3.7 ml/beat; p less than 0.05), and maximum increase in rate of change of left ventricular systolic pressure (control, 2475 +/- 200; infusion, 2088 +/- 216; recovery, 2487 +/- 243 mm Hg/sec; p less than 0.05) were also observed during infusion. No significant changes in total peripheral resistance or central venous pressure were noted, although the latter tended to fall during infusion. A similar pattern was observed in six pentobarbital-anesthetized dogs, except that infusion of atrial natriuretic factor did not induce bradycardia.(ABSTRACT TRUNCATED AT 250 WORDS)

The broad homeostatic role of natriuretic peptides

A brief non-inclusive review on natriuretic peptides (NP), their receptors, and their main functional properties is presented. The three main NP, atrial (ANP), brain (BNP) and C-type (CNP) are considered. Guanylyl cyclase receptors modulate all the known systemic effects of NP. Clearance receptors determine the metabolic disposal of NP and in this manner regulate their plasma levels and/or local tissue concentrations. Structure-function properties, and homeostatic properties of NP receptors are presented. ANP, which plays a major role in pressure-volume homeostasis, is discussed in relationship to its effects on renal hemodynamic and excretory functions, inhibition of the renin-angiotensin-aldosterone system, vasorelaxant, and third-spacing action. For BNP special attention is directed to its role as a negative modulator of ventricular remodeling, in view of its anti-hypertrophic, anti-fibrotic and anti-inflammatory effects in the heart. The major effect of CNP in promoting vertebral and longitudinal bone growth is briefly addressed. Finally, emphasis is placed on the recent discovery that ANP affects fat metabolism in humans due to its powerful lipolytic action.

Nitric oxide synthesis from L-arginine modulates renal vascular resistance in isolated perfused and intact rat kidneys

This study tested the effects on renal hemodynamics of blockage of nitric oxide (NO) synthesis from L-arginine with N G -methyl-L-arginine (L-NMMA) using both intact and isolated perfused rat kidneys. Infusion of L-NMMA into anesthetized rats increased the mean arterial pressure and reduced the glomerular filtration rate and renal plasma flow only when the renal perfucion pressure was maintained at control levels. In isolated kidneys, L-NMA increased vascular resistance; this was attenuated by coadministration of L-arginine.

Mast cells are required for the development of renal fibrosis in the rodent unilateral ureteral obstruction model

Mast cells are associated with inflammation and fibrosis. Whether they protect against or contribute to renal fibrosis is unclear. Based on our previous findings that mast cells can express and secrete active renin, and that angiotensin (ANG II) is profibrotic, we hypothesized that mast cells play a critical role in tubulointerstitial fibrosis. We tested this hypothesis in the 14-day unilateral ureteral obstruction (UUO) model in rats and mast cell-deficient (MCD) mice (WBB6F1-W/Wv) and their congenic controls (CC). In the 14-day UUO rat kidney, mast cell number is increased and they express active renin. Stabilizing mast cells in vivo with administration of cromolyn sodium attenuated the development of tubulointerstitial fibrosis, which was confirmed by measuring newly synthesized pepsin-soluble collagen and blind scoring of fixed trichrome-stained kidney sections accompanied by spectral analysis. Fibrosis was absent in UUO kidneys from MCD mice unlike that observed in the CC mice. Losartan treatment reduced the fibrosis in the CC UUO kidneys. The effects of mast cell degranulation and renin release were tested in the isolated, perfused kidney preparation. Mast cell degranulation led to renin-dependent protracted flow recovery. This demonstrates that mast cell renin is active in situ and the ensuing ANG II can modulate intrarenal vascular resistance in the UUO kidney. Collectively, the data demonstrate that mast cells are critical to the development of renal fibrosis in the 14-day UUO kidney. Since renin is present in human kidney mast cells, our work identifies potential targets in the treatment of renal fibrosis.

Renal vasoconstrictive effect of kinins mediated by B1-kinin receptors

The nature of the renal vascular actions of kinins, their dependence on prostaglandins and B1-kinin receptor responses were studied in functioning isolated perfused rat kidneys (IK). Lysylbradykinin (LBK), 0.28 and 0.7 microM, transiently decreased and then markedly increased the renal vascular resistance (RVR) in a sustained manner. Bradykinin (BK) at the same doses also had a transient vasorelaxant but not a sustained vasoconstrictive effect. The inactivation of LBK and BK by the IK did not account for the transient nature of their vasorelaxant effect. Indomethacin (5 microM) markedly blunted LBK-induced decrease but not increase in RVR. The B1-kinin receptor agonist desArg9-BK (0.4-1.0 microM) did not decrease RVR but, as LBK, markedly increased RVR in a dose-related manner. The B1-kinin receptor antagonist [Leu8]desArg9-BK had no effect on its own but inhibited the desArg9-BK-induced vasoconstriction in a stoichiometric manner. This antagonist at 4.0 microM also completely abolished the vasoconstrictive effect of 0.7 microM LBK, whereas it potentiated and prolonged its vasorelaxant effect. The results demonstrate that kinins, particularly LBK, have bimodal effects on the renal vascular resistance of the isolated perfused rat kidney. The vasorelaxant effect is at least partly mediated by prostaglandins whereas the vasoconstrictive effect of LBK and/or its renal metabolites has the typical character of a B1-kinin receptor response. It is postulated that B1-kinin receptor responses may be of importance in the generation and/or maintenance of renal vasoconstriction in disease states which lead to renal failure.

Role of increased glomerular filtration rate in atrial natriuretic factor-induced natriuresis in the rat

One of the major renal hemodynamic actions of atrial natriuretic factor (ANF) is to increase glomerular filtration rate (GFR). To assess the role of this effect on ANF-induced natriuresis (UNaV), diuresis (V) and kaliuresis (UKV) we performed late clamp experiments in six rats. After control periods (C), synthetic ANF (auriculin A) was infused i.v. (2 micrograms X min-1/kg body wt) throughout the experiment (150 min). After pre-clamp periods, the perfusion pressure of the left kidney (LK) was reduced to 75-80 mmHg. The right kidney (RK) served as a time control. In LK, before the late clamp, ANF increased (p less than 0.01) GFR from 1.5 +/- 0.1 to 1.8 +/- 0.1 ml/min, V from 17 +/- 5 to 53 +/- 5 microliters/min, and UNaV from 2.1 +/- 0.6 to 10.0 +/- 0.9 microEq/min. Almost identical increases occurred in the RK. The late clamp returned all parameters in LK to C values (p greater than 0.05): GFR to 1.4 +/- 0.1 ml/min, V to 6.3 +/- 1.2 microliter/min, and UNaV to 1.0 +/- 0.3 microEq/min. The late clamp also reversed the ANF-induced increase in UKV. In the RK, GFR (1.8 +/- 0.1 ml/min), V (38 +/- 4 microliter/min) and UNaV (7.8 +/- 0.8 microEq/min) remained elevated (p less than 0.01 vs. C) to the end of the experiment. These data demonstrate that upon return of GFR to control levels, the ANF-induced diuresis, natriuresis and kaliuresis is abolished. The results support our previous view that the increase in GFR together with a decrease in inner-medullary hypertonicity account wholly or in great part for the natriuretic action of ANF.

FUNCTIONAL PROPERTIES AND DYNAMICS OF NATRIURETIC PEPTIDE RECEPTORS

Since the discovery of atrial natriuretic factor (ANF or ANP) in 1981 by De Bold et al. (1), more than 6500 articles on the subject have appeared in the literature. For an extensive review on ANP and its receptors, the reader is directed to Reference 2. In this short review, we will consider in a noninclusive manner the dynamics and functional properties of natriuretic peptide receptors. ANP is a 28-amino acid polypeptide hormone secreted mainly by the heart atria in response to atrial stretch that results from an increase in central venous pressure, or systemic and pulmonary hypertension. ANP is a member of a family of natriuretic peptides, which include B-type or brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP), and urodilatin. The physiological role of BNP, CNP, and urodilatin remains essentially unknown. The main actions of ANP in the mammalian organism are geared toward the regulation of volumepressure homeostasis (2,3). In the kidney, ANP exerts hemodynamic and tubular actions that lead to diuresis and natriuresis. In the cardiovascular system, ANP acts as a strong antagonist of vasoconstriction and also decreases plasma volume, a combination of events that reduces blood pressure. The decrease in plasma volume is due to a shift of fluid from the intravascular to the interstitial compartment brought about by an increase in hydraulic permeability of systemic capillaries. ANP is also a powerful functional antagonist of the renin-angiotensin-aldosterone system through inhibiting renin secretion by the kidney, aldosterone synthesis by the adrenal, and by antagonizing all known peripheral actions of angiotensin 11. The inhibition of the activity and effects of renin-angiotensinaldosterone contributes substantially to the cardiovascular effects of ANP. Several in vivo studies in the past 8 years, including studies on inhibition of ANP metabolism by blockade of C receptors, administration of specific guanylyl cyclase receptor antagonists, and, more recently, studies on gene deletion of ANP and guanylyl cyclase type A receptors in mice, conclusively demonstrated that ANP, through guanylyl cyclase type A receptors, play a major role in pressurevolume homeostasis (2-5).

Conversion of T-kinin to bradykinin by the rat kidney

Isolated rat kidneys were perfused with T-kinin (TK, Ile-Ser-BK) and bradykinin (BK). HPLC analysis of perfusate samples taken at 2-10 min during the TK perfusion (0.5 nmol/mL initial concentration) showed two peptide peaks, the first one eluting at 14.42 min, the same retention time for standard BK, and the second at 16.20 min, corresponding to that of TK. When BK (0.5 nmol/mL) was perfused, only its corresponding peak was obtained although total BK recovery was reduced quickly, as expected. Using both HPLC analysis and a kinin bioassay on the isolated guinea pig ileum, it was found that 12% of the added TK was converted to BK during the first perfusion cycle (2 min). While the BK recovered (12-14% from the initial TK concentration) was maintained at a similar proportion between the 2nd and the 10th min of perfusion, the rate of TK disappearance, as well as its full recovery from the perfusate, indicated further fragmentation of peptides during kinin perfusion. In the presence of 5 microM DL-mercaptomethyl-3-guanidino-ethylthiopropanoic acid (Mergetpa), an inhibitor of plasma carboxypeptidase N (EC 3.4.17.3), the rate of conversion of TK to BK was not affected. On the other hand, the kinase II inhibitor bradykinin potentiating peptide 9a (BPP9a) increased both the proportion of TK converted to BK and the disappearance rate of TK from the perfusate. In the presence of BPP9a, the rate of BK production increased from 1.5 +/- 0.2 to 7.6 +/- 0.9 nmol/min. Furthermore, the recovery of BK was reduced during the first 2 min of perfusion to 7.6% and the conversion rate to 0.9 nmol/min when TK was perfused into the kidney in the presence of 10 microM bestatin, a known inhibitor of aminopeptidases. These data indicate that in the kidney TK is converted to BK, probably by aminopeptidase M, thus suggesting that BK is, in fact, an additional and functional kinin, inducing physiological and/or pathophysiological effects in the rat kidney in which TK is the main kinin released.