Sex differences in pressure diuresis/natriuresis in rabbits

SARS CoV2 infection _The longevity study perspectives

Like other infectious diseases, COVID-19 shows a clinical outcome enormously variable, ranging from asymptomatic to lethal. In Italy, like in other countries, old male individuals, with one or more comorbidity, are the most susceptible group, and show, consequently, the highest mortality, and morbidity, including lethal respiratory distress syndrome, as the most common complication. In addition, another extraordinary peculiarity, that is a surprising resistance to COVID-19, characterizes some Italian nonagenarians/centenarians. Despite having the typical COVID-19 signs and/or symptoms, such exceptional individuals show a surprising tendency to recover from illness and complications. On the other hand, long-lived people have an optimal performance of immune system related to an overexpression of anti-inflammatory variants in immune/inflammatory genes, as demonstrated by our and other groups. Consequently, we suggest long-lived people as an optimal model for detecting genetic profiles associated with the susceptibility and/or protection to COVID-19, to utilize as potential pharmacological targets for preventing or reducing viral infection in more vulnerable individuals.

Animal Models of Hypertension: A Scientific Statement From the American Heart Association

Hypertension is the most common chronic disease in the world, yet the precise cause of elevated blood pressure often cannot be determined. Animal models have been useful for unraveling the pathogenesis of hypertension and for testing novel therapeutic strategies. The utility of animal models for improving the understanding of the pathogenesis, prevention, and treatment of hypertension and its comorbidities depends on their validity for representing human forms of hypertension, including responses to therapy, and on the quality of studies in those models (such as reproducibility and experimental design). Important unmet needs in this field include the development of models that mimic the discrete hypertensive syndromes that now populate the clinic, resolution of ongoing controversies in the pathogenesis of hypertension, and the development of new avenues for preventing and treating hypertension and its complications. Animal models may indeed be useful for addressing these unmet needs.

Sex-specific differences in hypertension and associated cardiovascular disease

Although intrinsic mechanisms that regulate arterial blood pressure (BP) are similar in men and women, marked variations exist at the molecular, cellular and tissue levels. These physiological disparities between the sexes likely contribute to differences in disease onset, susceptibility, prevalence and treatment responses. Key systems that are important in the development of hypertension and cardiovascular disease (CVD), including the sympathetic nervous system, the renin-angiotensin-aldosterone system and the immune system, are differentially activated in males and females. Biological age also contributes to sexual dimorphism, as premenopausal women experience a higher degree of cardioprotection than men of similar age. Furthermore, sex hormones such as oestrogen and testosterone as well as sex chromosome complement likely contribute to sex differences in BP and CVD. At the cellular level, differences in cell senescence pathways may contribute to increased longevity in women and may also limit organ damage caused by hypertension. In addition, many lifestyle and environmental factors - such as smoking, alcohol consumption and diet - may influence BP and CVD in a sex-specific manner. Evidence suggests that cardioprotection in women is lost under conditions of obesity and type 2 diabetes mellitus. Treatment strategies for hypertension and CVD that are tailored according to sex could lead to improved outcomes for affected patients.

The "his and hers" of the renin-angiotensin system

Sex differences exist in the regulation of arterial pressure and renal function by the renin-angiotensin system (RAS). This may in part stem from a differential balance in the pressor and depressor arms of the RAS. In males, the ACE/AngII/AT(1)R pathways are enhanced, whereas, in females, the balance is shifted towards the ACE2/Ang(1-7)/MasR and AT(2)R pathways. Evidence clearly demonstrates that premenopausal women, as compared to aged-matched men, are protected from renal and cardiovascular disease, and this differential balance of the RAS between the sexes likely contributes. With aging, this cardiovascular protection in women is lost and this may be related to loss of estrogen postmenopause but the possible contribution of other sex hormones needs to be further examined. Restoration of these RAS depressor pathways in older women, or up-regulation of these in males, represents a therapeutic target that is worth pursuing.

Chronic maternal hypertension characterized by renal dysfunction is associated with reduced placental blood flow during late gestation in rabbits

Maternal hypertension associated with renal disease is a common pregnancy complication. Previously, we have shown in a rabbit model of mild hypertension that offspring from hypertensive mothers have increased blood pressure as adults. In human pregnancy, hypertension has been associated with decreased utero-placental blood flow. The aim of this study was to determine placental blood flow (PBF) in mild (2-kidney-1-wrapped; 2K-1W) and moderate (2-kidney-2-wrapped; 2K-2W) rabbit models of maternal hypertension. We hypothesized that PBF would be inversely related to the severity of the hypertension. PBF and renal blood flow (RBF) were measured using microspheres on day 28 of a 32-day gestation, in normotensive (sham), 2K-1W, and 2K-2W hypertensive groups. Mean arterial pressure (MAP, ∼7 mmHg, P < 0.05) was increased, and RBF (∼35%, P < 0.05) was reduced in the 2K-1W and 2K-2W (MAP ∼20 mmHg, P < 0.01; RBF ∼53%, P < 0.05) groups compared with the sham group. In the 2K-1W group, PBF fell by ∼12% ( P = 0.08) and fetal-to-placental weight ratio increased by ∼12% ( P < 0.01) compared with the sham group, reflecting an increase in the functional capacity of the placenta to deliver nutrients to the fetus. In the 2K-2W group, PBF decreased ∼51% ( P < 0.05) compared with the sham group, without changes in placental efficiency. Thus, in late gestation, placental blood flow was significantly reduced in the moderate hypertension group, without accompanying changes in fetal or placental weight or placental efficiency. In contrast, mild hypertension resulted in an increase in placental efficiency, without significant changes in placental blood flow. These findings suggest that mild and moderate hypertension may alter placental delivery of nutrients via differing mechanisms dependent upon the severity of the hypertension.

Estrogen receptors in the kidney: lessons from genetically altered mice

BACKGROUND

Sex differences in human and animal models of kidney disease suggest that estrogen receptor (ER)-mediated events may modulate these processes. Genetically altered mice lacking one or both ERs provide a powerful tool to study these phenomena.

OBJECTIVE

This article examines sex differences in the kidney, particularly the role of ERs.

METHODS

To identify pertinent studies in genetically altered mice, a literature search was conducted on the MEDLINE database from January 1966 to July 2007, using the search terms estrogen receptor, kidney, and mice. Our group examined the effect of the ER-alpha knockout genotype on the kidney in streptozotocin-induced diabetes mellitus and compensatory kidney growth after uninephrectomy.

RESULTS

Female mice lacking ERa had reduced renal growth, including glomerular enlargement after 2 weeks of streptozotocin-induced diabetes mellitus and compensatory kidney growth 48 hours after uninephrectomy.

CONCLUSION

ER-mediated events influence kidney growth and disease in female mice.

Effects of dietary protein restriction on nephron number in the mouse

In rats, maternal protein restriction reduces nephron endowment and often leads to adult hypertension. Sex differences in these responses have been identified. The molecular and genetic bases of these phenomena can best be identified in a mouse model, but effects of maternal protein restriction on kidney development have not been examined in mice. Therefore, we determined how combined prenatal and postnatal protein restriction in mice affects organ weight, glomerular number and dimensions, and renal expression of angiotensin receptor mRNA, in both male and female offspring. C57/BL6/129sv mice received either a normal (20% wt/wt; NP) or low (9% wt/wt; LP) protein diet during gestation and postnatal life. Offspring were examined at postnatal day 30. Protein restriction retarded growth of the kidney, liver, spleen, heart, and brain. All organs except the brain weighed less in female than male offspring. Protein restriction increased normalized (to body weight) brain weight, with females having relatively heavier brains than males. The effects of protein restriction were not sex dependent, except that normalized liver weight was reduced in males but increased in females. Glomerular volume, but not number, was greater in female than in male mice. Maternal protein restriction reduced nephron endowment similarly in male and female mice. Renal expression of AT1A receptor mRNA was approximately sixfold greater in female than male NP mice, but similar in male LP and female LP mice. We conclude that maternal protein restriction reduces nephron endowment in mice. This effect provides a basis for future studies of developmental programming in the mouse.

ANG II type 2 receptors and neural control of intrarenal blood flow

We tested the hypothesis that activation of angiotensin type 2 (AT(2)) receptors, by both exogenous and endogenous ANG II, modulates neurally mediated vasoconstriction in the renal cortical and medullary circulations. Under control conditions in pentobarbital-anesthetized rabbits, electrical stimulation of the renal nerves (RNS; 0.5-8 Hz) reduced renal blood flow (RBF; -88 +/- 3% at 8 Hz) and cortical perfusion (CBF; -92 +/- 2% at 8 Hz) more than medullary perfusion (MBF; -67 +/- 6% at 8 Hz). Renal arterial infusion of ANG II, at a dose titrated to reduce RBF by approximately 40-50% (5-50 ng.kg(-1).min(-1)) blunted responses of MBF to RNS, without significantly affecting responses of RBF or CBF. Subsequent administration of PD123319 (1 mg/kg plus 1 mg.kg(-1).h(-1)) during continued renal arterial infusion of ANG II did not significantly affect responses of RBF or CBF to RNS but enhanced responses of MBF, so that they were similar to those observed under control conditions. In contrast, administration of PD123319 alone blunted responses of CBF and MBF to RNS. Subsequent renal arterial infusion of ANG II in PD123319-pretreated rabbits restored CBF responses to RNS back to control levels. In contrast, ANG II infusion in PD123319-pretreated rabbits did not alter MBF responses to RNS. These data indicate that exogenous ANG II can blunt neurally mediated vasoconstriction in the medullary circulation through activation of AT(2) receptors. However, AT(2)-receptor activation by endogenous ANG II appears to enhance neurally mediated vasoconstriction in both the cortical and medullary circulations.

Angiotensin II and nitric oxide in neural control of intrarenal blood flow

We investigated the roles of the renin-angiotensin system and the significance of interactions between angiotensin II and nitric oxide, in responses of regional kidney perfusion to electrical renal nerve stimulation (RNS) in pentobarbital sodium-anesthetized rabbits. Under control conditions, RNS (0.5–8 Hz) reduced total renal blood flow (RBF; −89 ± 3% at 8 Hz) and cortical perfusion (CBF; −90 ± 2% at 8 Hz) more than medullary perfusion (MBF; −55 ± 5% at 8 Hz). Angiotensin II type 1 (AT1)-receptor antagonism (candesartan) blunted RNS-induced reductions in RBF ( P = 0.03), CBF ( P = 0.007), and MBF ( P = 0.04), particularly at 4 and 8 Hz. Nitric oxide synthase inhibition with NG-nitro-l-arginine (l-NNA) enhanced RBF ( P = 0.003), CBF ( P = 0.001), and MBF ( P = 0.03) responses to RNS, particularly at frequencies of 2 Hz and less. After candesartan pretreatment, l-NNA significantly enhanced RNS-induced reductions in RBF ( P = 0.04) and CBF ( P = 0.007) but not MBF ( P = 0.66). Renal arterial infusion of angiotensin II (5 ng·kg−1·min−1) selectively enhanced responses of MBF to RNS in l-NNA-pretreated but not in vehicle-pretreated rabbits. In contrast, greater doses of angiotensin II (5–15 ng·kg−1·min−1) blunted responses of MBF to RNS in rabbits with intact nitric oxide synthase. These results suggest that endogenous angiotensin II enhances, whereas nitric oxide blunts, neurally mediated vasoconstriction in the renal cortical and medullary circulations. In the renal medulla, but not the cortex, angiotensin II also appears to be able to blunt neurally mediated vasoconstriction.

Effect of renal perfusion pressure on responses of intrarenal blood flow to renal nerve stimulation in rabbits

1. We investigated how sympathetic nerve activity and renal perfusion pressure (RPP) interact in controlling renal haemodynamics in pentobarbitone-anaesthetized rabbits. 2. Renal blood flow (RBF) was reduced by electrical renal nerve stimulation (0.5-8 Hz), with RPP set using an extracorporeal circuit to 65, 100 and 135 mmHg. 3. Responses of RBF and cortical laser Doppler flux to renal nerve stimulation were blunted by increased RPP. For example, 4 Hz stimulation reduced RBF by 68 +/- 7% with baseline perfusion pressure approximately 65 mmHg, but only by 22 +/- 3% at approximately 135 mmHg. Medullary laser Doppler flux was less responsive than cortical laser Doppler flux to renal nerve stimulation and its response was not dependent on perfusion pressure. 4. When perfusion pressure was clamped at its baseline level during renal nerve stimulation, responses of RBF and cortical laser Doppler flux, but not medullary laser Doppler flux, were still blunted with increased baseline perfusion pressure. 5. A frequency rich stimulus was applied to assess the effects of perfusion pressure on dynamic neural control of RBF. Renal blood flow responded similarly at each level of perfusion pressure, as a low-pass filter with a pure time delay. 6. Our results suggest that, in the rabbit extracorporeal circuit model, increased RPP blunts the ability of steady state renal nerve stimulation to reduce cortical, but not medullary perfusion. However, in this model the level of RPP appears to have little impact on dynamic neural control of RBF.

Prostaglandins and nitric oxide in regional kidney blood flow responses to renal nerve stimulation

We examined the roles of cyclooxygenase products and of interactions between the cyclooxygenase and nitric oxide systems in the mechanisms underlying the relative insensitivity of medullary perfusion to renal nerve stimulation (RNS) in anaesthetized rabbits. To this end we examined the effects of ibuprofen and N(G)-nitro-L: -arginine (L-NNA), both alone and in combination, on the responses of regional kidney perfusion to RNS. Under control conditions, RNS produced frequency-dependent reductions in total renal blood flow (RBF; -82+/-3% at 6 Hz), cortical laser-Doppler flux (CLDF; -84+/-4% at 6 Hz) and, to a lesser extent, medullary laser-Doppler flux (MLDF; -46+/-7% at 6 Hz). Ibuprofen did not affect these responses significantly, suggesting that cyclooxygenase products have little net role in modulating renal vascular responses to RNS. L-NNA enhanced RBF (P=0.002), CLDF (P=0.03) and MLDF (P=0.03) responses to RNS. As we have shown previously, this effect of L-NNA was particularly prominent for MLDF at RNS frequencies < or = 1.5 Hz. Subsequent administration of ibuprofen, in L-NNA-pretreated rabbits, did not affect responses to RNS significantly. We conclude that counter-regulatory actions of NO, but not of prostaglandins, partly underlie the relative insensitivity of medullary perfusion to renal nerve activation.

Modulation of V1-receptor-mediated renal vasoconstriction by epoxyeicosatrienoic acids

This study examined the effects of renal arterial infusion of a selective cytochrome P-450 epoxygenase inhibitor, N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH; 2 mg/kg plus 1.5 mg·kg−1·h−1), on renal hemodynamic responses to infusions of [Phe2,Ile3,Orn8]vasopressin and ANG II into the renal artery of anesthetized rabbits. MS-PPOH did not affect basal renal blood flow (RBF) or cortical or medullary blood flow measured by laser-Doppler flowmetry (CLDF/MLDF). In vehicle-treated rabbits, [Phe2,Ile3,Orn8]vasopressin (30 ng·kg−1·min−1) reduced MLDF by 62 ± 7% but CLDF and RBF were unaltered. In MS-PPOH-treated rabbits, RBF and CLDF fell by 51 ± 8 and 59 ± 13%, respectively, when [Phe2,Ile3,Orn8]vasopressin was infused. MS-PPOH had no significant effects on the MLDF response to [Phe2,Ile3,Orn8]vasopressin (43 ± 9% reduction). ANG II (20 ng·kg−1·min−1) reduced RBF by 45 ± 10% and CLDF by 41 ± 14%, but MLDF was not significantly altered. MS-PPOH did not affect blood flow responses to ANG II. Formation of epoxyeicosatrienoic acids (EETs) and dihydroxyeicosatrienoic acids (DiHETEs) was 49% lower in homogenates prepared from the renal cortex of MS-PPOH-treated rabbits than from vehicle-treated rabbits. MS-PPOH had no effect on the renal formation of 20-hydroxyeicosatetraenoic acid (20-HETE). Incubation of renal cortical homogenates from untreated rabbits with [Phe2,Ile3,Orn8]vasopressin (0.2–20 ng/ml) did not affect formation of EETs, DiHETEs, or 20-HETE. These results do not support a role for de novo EET synthesis in modulating renal hemodynamic responses to ANG II. However, EETs appear to selectively oppose V1-receptor-mediated vasoconstriction in the renal cortex but not in the medullary circulation and contribute to the relative insensitivity of medullary blood flow to V1-receptor activation.

α-Adrenoceptor subtypes mediating regional kidney blood flow responses to renal nerve stimulation

The mechanisms underlying the relative insensitivity of the renal medullary circulation to renal sympathetic nerve stimulation (RNS) remain unknown. Therefore, we tested the effects of systemic alpha(1)- and alpha(2)-adrenoceptor blockade on responses to electrical RNS in pentobarbitone anaesthetized rabbits. Renal blood flow (RBF), cortical laser Doppler flux (CLDF), and to a lesser extent medullary LDF (MLDF) were reduced by RNS in a frequency-dependent manner. Prazosin decreased responses of RBF and CLDF, but not MLDF, to RNS. For example, during the control period 4 Hz stimulation reduced RBF, CLDF and MLDF by 85+/-3%, 89+/-2%, and 20+/-12%, respectively, but after prazosin, corresponding responses were 39+/-3%, 42+/-5% and 28+/-7%, respectively. Prazosin markedly blunted pressor and renal vasoconstrictor responses to intravenous phenylephrine, without altering pressor responses to intravenous xylazine. Rauwolscine enhanced renal vasoconstrictor responses to RNS, although this was statistically significant for RBF and CLDF but not MLDF. For example, during the control period 2 Hz stimulation reduced RBF, CLDF and MLDF by 63+/-7%, 58+/-7%, and 29+/-17%, respectively, and after rauwolscine, corresponding responses were 83+/-4%, 87+/-1%, and 53+/-12%, respectively. Rauwolscine markedly blunted renal vasoconstrictor responses to renal arterial guanabenz, but not phenylephrine. These data suggest that alpha(1)-adrenoceptors contribute to RNS-induced vasoconstriction in the renal cortex, but contribute less in vascular elements controlling medullary perfusion. Activation of alpha(2)-adrenoceptors appears to blunt RNS-induced renal vasoconstriction, but this mechanism does not underlie the relative insensitivity of medullary perfusion to RNS.

Lipoxygenase and cyclo-oxygenase products in the control of regional kidney blood flow in rabbits

1. The aim of the present study was to examine the roles of cyclo-oxygenase (COX)- and lipoxygenase (LOX)-dependent arachidonate signalling cascades in the control of regional kidney blood flow. 2. In pentobarbitone-anaesthetized rabbits treated with NG-nitro-l-arginine and glyceryl trinitrate to 'clamp' nitric oxide, we determined the effects of ibuprofen (a COX inhibitor) and esculetin (a LOX inhibitor) on resting systemic and renal haemodynamics and responses to renal arterial infusions of vasoconstrictors. 3. Ibuprofen increased mean arterial pressure (14 +/- 5%) and reduced medullary laser Doppler flux (MLDF; 26 +/- 6%) when administered with esculetin. A similar pattern of responses was observed when ibuprofen was given alone, although the reduction in MLDF was not statistically significant. Esculetin tended to increase renal blood flow (RBF; 16 +/- 7%) and MLDF (28 +/- 13%) when given alone, but not when combined with ibuprofen. 4. After vehicle, renal arterial infusions of noradrenaline, angiotensin II and endothelin-1 reduced RBF and cortical laser Doppler flux (CLDF), but not MLDF. In contrast, renal arterial [Phe2,Ile3,Orn8]-vasopressin reduced MLDF but not RBF or CLDF. Ibuprofen alone did not significantly affect these responses. Esculetin, when given alone, but not when combined with ibuprofen, enhanced noradrenaline-induced renal vasoconstriction. In contrast, esculetin did not significantly affect responses to [Phe2,Ile3,Orn8]-vasopressin, angiotensin II or endothelin-1. 5. We conclude that COX products contribute to the maintenance of arterial pressure and renal medullary perfusion under 'nitric oxide clamp' conditions, but not to renal haemodynamic responses to the vasoconstrictors we tested. Lipoxygenase products may blunt noradrenaline-induced vasoconstriction, but our observations may, instead, reflect LOX-independent effects of esculetin.

Adult rabbit offspring of mothers with secondary hypertension have increased blood pressure

Preexisting chronic hypertension complicates up to 5% of pregnancies and is associated with an increased risk of low-birth-weight babies. Studies suggest that an adverse intrauterine environment leading to low birth weight is linked to an increased risk of cardiovascular disease, including hypertension, in the adult. In this study, the blood pressure of offspring from mothers with hypertension were followed up into adulthood. Two-kidney, 1-wrapped hypertension was induced in 7 female rabbits; 5 other rabbits underwent sham surgery. Four weeks later, rabbits were mated, at which time mean arterial pressure was 118+/-3 and 87+/-5 mm Hg in the hypertensive and sham groups, respectively (P<0.001). The blood pressure of 30-week-old females was 89+/-2 mm Hg in the offspring of hypertensive (n=14) and 79+/-1 mm Hg in the offspring of normotensive (n=13) mothers (P<0.005). Also, plasma renin activity was significantly lower in the female offspring of hypertensive mothers at 10 weeks of age (P<0.05), suggesting that development of the renin-angiotensin system was altered. In contrast, male offspring from hypertensive and normotensive mothers had similar mean arterial pressure and plasma renin activity. In conclusion, maternal secondary hypertension can "program" hypertension in female adult offspring. The results also suggest that there are gender-specific differences in sensitivity to altered in utero environmental influences.

Regional responsiveness of renal perfusion to activation of the renal nerves

We tested for regional differences in perfusion responses, within the renal medulla and cortex, to renal nerve stimulation in pentobarbital sodium-anesthetized rabbits. Laser-Doppler flux (LDF) was monitored at various depths below the cortical surface (1-15 mm). Basal cortical LDF (1-3 mm, approximately 200-450 U) was greater than medullary LDF (5-15 mm, approximately 70-160 U), but there were no statistically significant differences in basal LDF within these regions. The background LDF signal during aortic occlusion was similar in the cortex (2 mm, 31 U) and outer medulla (7 mm, 31 U), but slightly greater in the inner medulla (12 mm, 44 U). During electrical stimulation of the renal nerves (0.5-8 Hz), cortical LDF and total renal blood flow were similarly progressively reduced with increasing stimulus frequency. Medullary LDF (measured between 5 and 15 mm) was overall less responsive than cortical LDF. For example, 4-Hz stimulation reduced inner medullary LDF (9 mm) by 19 +/- 6% but reduced cortical LDF (1 mm) by 54 +/- 11%. However, medullary LDF responses to nerve stimulation were similar at all depths measured. Our results indicate that while the vascular elements controlling medullary perfusion are less sensitive to the effects of electrical stimulation of the renal nerves than are those controlling cortical perfusion, sensitivity within these vascular territories appears to be relatively homogeneous.

Diabetic kidney disease: impact of puberty

Puberty accelerates microvascular complications of diabetes mellitus, including nephropathy. Animal studies confirm a different renal hypertrophic response to diabetes before and after puberty, probably due to differences in the production of transforming growth factor-beta (TGF-beta). Many of the complex physiological changes during puberty could affect potentially pathogenic mechanisms of diabetic kidney disease. Increased blood pressure, activation of the growth hormone-insulin-like growth factor I axis, and production of sex steroids could all play a role in pubertal susceptibility to diabetic renal hypertrophy and nephropathy. These factors may influence the effects of hyperglycemia and several systems that ultimately control TGF-beta production, including the renin-angiotensin system, cellular redox systems, the polyol pathway, and protein kinase C. These phenomena may also explain gender differences in kidney function and incidence of end-stage renal disease. Normal changes during puberty, when coupled with diabetes and superimposed on a genetically susceptible milieu, are capable of accelerating diabetic hypertrophy and microvascular lesions. A better understanding of these processes may lead to new treatments to prevent renal failure in diabetes mellitus.

Effects of indomethacin on responses of regional kidney perfusion to vasoactive agents in rabbits

1. To determine whether differential release of products of arachidonic acid metabolism, via the cyclo-oxygenase pathway, underlies the diversity of responses of regional kidney perfusion to vasoactive agents, we tested the effects of intravenous indomethacin on responses to renal arterial bolus doses of vasoactive agents in pentobarbitone-anaesthetized rabbits. 2. Total renal blood flow (RBF) and regional kidney perfusion were determined by transit time ultrasound flowmetry and laser-Doppler flowmetry, respectively. 3. Responses of regional kidney blood flow to vasoactive agents were diverse: noradrenaline reduced cortical but not medullary perfusion, [Phe 2,Ile 3,Orn 8]-vasopressin reduced medullary perfusion more than cortical perfusion, endothelin-1 and angiotensin II increased medullary perfusion in the face of reduced cortical perfusion, while acetylcholine, bradykinin and the nitric oxide donor methylamine hexamethylene methylamine (MAHMA) NONOate all increased both cortical and medullary perfusion. 4. Indomethacin administration was followed by reductions in total RBF (17 +/- 6%), cortical perfusion (13 +/- 5%) and medullary perfusion (40 +/- 8%). Angiotensin II- and endothelin-1-induced increases in medullary perfusion were abolished by indomethacin, but indomethacin had no significant effects on responses of regional kidney perfusion to acetylcholine, bradykinin, MAHMA NONOate, noradrenaline and [Phe 2,Ile 3,Orn 8]-vasopressin. 5. Our results suggest that vasodilator cyclo-oxygenase products contribute to the maintenance of resting renal vascular tone, particularly in vascular elements controlling medullary perfusion. Cyclo-oxygenase products also appear to mediate endothelin-1- and angiotensin II-induced increases in medullary perfusion. However, regionally specific engagement of cyclo-oxygenase-dependent arachidonic acid metabolism does not appear to contribute to the differential effects of noradrenaline and [Phe 2,Ile 3,Orn 8]-vasopressin on cortical and medullary perfusion.

Nitric oxide in responses of regional kidney blood flow to vasoactive agents in anesthetized rabbits

To determine whether differential release of nitric oxide underlies the diversity of regional kidney blood flow responses to vasoactive agents, this study examined how nitric oxide synthase blockade with IV N(G)-nitro-L-arginine (L-NNA), and also IV L-NNA plus co-infusion of glyceryl trinitrate, affected responses to renal arterial boluses and infusions of vasoactive agents. L-NNA, but not vehicle, or L-NNA plus glyceryl trinitrate, increased mean arterial pressure (35%) and reduced renal blood flow (20%), cortical perfusion (11%), and medullary perfusion (54%). L-NNA plus glyceryl trinitrate, but not L-NNA alone, blunted renal vasodilatation in response to boluses of bradykinin and acetylcholine, abolished increased medullary perfusion after bolus angiotensin II, and enhanced reductions in medullary perfusion, and to a lesser extent those in renal blood flow and cortical perfusion, during norepinephrine infusion. Neither L-NNA, nor L-NNA plus glyceryl trinitrate, affected responses to infusions of angiotensin II, [Phe(2),Ile(3),Orn(8)]-vasopressin, or endothelin-1. The data indicate roles for nitric oxide in angiotensin II-induced increases in medullary perfusion and in protecting medullary perfusion from norepinephrine-induced vasoconstriction. However, differential engagement of nitric oxide synthase cannot completely account for the diversity of responses of regional kidney perfusion to vasoactive agents. Effects of nitric oxide synthase blockade on renal vascular responses to vasoactive agents were revealed only when glyceryl trinitrate was co-infused to restore resting nitrergic vasodilator tone. This may reflect interactions between nitric oxide and other vasodilator mediators, in modulating renal hemodynamic responses to vasoactive agents.

Renal medullary interstitial infusion is a flawed technique for examining vasodilator mechanisms in anesthetized rabbits

INTRODUCTION

In rats, medullary interstitial (IMI) infusion is a useful technique for selective delivery of pharmacological agents to the renal medulla, in both acute and chronic experimental settings. We examined the feasibility of using this technique for delivery of vasodilators in rabbits, since this larger species would provide a number of advantages, particularly in long-term studies of circulatory control.

METHODS

Rabbits were anesthetized with pentobarbitone and artificially ventilated. Catheters were placed in a side branch of the renal artery and/or the renal medullary interstitium. Renal blood flow (RBF) was determined by transit-time ultrasound flowmetry, and blood flow in the cortex and medulla was estimated by laser Doppler flowmetry.

RESULTS

Pilot studies showed that renal arterial (IRA) infusions of bradykinin (10-300 ng/kg/min) and adenosine (1-10 ng/kg/min) produced only transient renal vasodilatation. IRA infusions of methylamine hexamethylene methylamine (MAHMA) NONOate (100-1000 ng/kg/min) and acetylcholine (10-250 ng/kg/min) produced dose-dependent and sustained increases in RBF and reductions in arterial pressure at the highest doses. However, IMI infusion of the same doses did not consistently increase medullary laser Doppler flux (MLDF). After IRA MAHMA NONOate and IMI acetylcholine, RBF fell to below its resting level. IRA boluses of acetylcholine (10-1250 ng/kg), bradykinin (2-250 ng/kg), and MAHMA NONOate (100-3000 ng/kg) dose-dependently increased RBF and CLDF and MLDF.

DISCUSSION

We had previously validated the IMI infusion technique for intramedullary delivery of vasoconstrictors in rabbits. Our present results indicate that this technique has limited application for delivery of vasodilator agents, in part because counterregulatory vasoconstrictor mechanisms are activated.

Dominance of pressure natriuresis in acute depressor responses to increased renal artery pressure in rabbits and rats

Increasing renal artery pressure (RAP) activates pressure diuresis/natriuresis and inhibits renal renin release. There is also evidence that increasing RAP stimulates release of a putative depressor hormone from the renal medulla, although this hypothesis remains controversial. We examined the relative roles of these antihypertensive mechanisms in the acute depressor responses to increased RAP in anaesthetized rabbits and rats. In rabbits, an extracorporeal circuit was established which allows RAP to be set and controlled without direct effects on systemic haemodynamics. When RAP was maintained at approximately 65 mmHg, cardiac output (CO) and mean arterial pressure (MAP) did not change significantly. In contrast, when RAP was increased to approximately 160 mmHg, CO and MAP fell 20 +/- 5 % and 36 +/- 5 %, respectively, over 30 min. Urine flow also increased more than 28-fold when RAP was increased. When compound sodium lactate was infused intravenously at a rate equal to urine flow, neither CO nor MAP fell significantly in response to increased RAP. In 1 kidney-1 clip hypertensive rats, MAP fell by 54 +/- 10 mmHg over a 2 h period after unclipping. In rats in which isotonic NaCl was administered intravenously at a rate equal to urine flow, MAP did not change significantly after unclipping (-14 +/- 9 mmHg). Our results suggest that the depressor responses to increasing RAP in these experimental models are chiefly attributable to hypovolaemia secondary to pressure diuresis/natruresis. These models therefore appear not to be bioassays for release of a putative renal medullary depressor hormone.

Effects of ET(A) - and ET(B)-receptor antagonists on regional kidney blood flow, and responses to intravenous endothelin-1, in anaesthetized rabbits

OBJECTIVE

To determine the roles of endothelin (ET)-receptor subtypes in the effects of exogenous and endogenous ETs on regional kidney blood flow in anaesthetized rabbits.

DESIGN AND METHODS

The effects on regional kidney blood flow of the ET(A) antagonist BQ610, and the ET(B) antagonist BQ788, were tested. We also examined the effects of intravenous and renal arterial bolus doses of ET-1, and how these responses are modified by pretreatment with BQ610 and BQ788.

RESULTS

BQ610 reduced mean arterial pressure (MAP, 3%), and increased total renal blood flow (RBF, 10%), cortical perfusion (CBF, 11%) and medullary perfusion (MBF, 16%). BQ788 increased MAP (6%) and reduced RBF (16%) and CBF (13%) but not MBF. The effects of BQ788 were abolished by pretreatment with BQ610. Intravenous ET-1 (300 ng/kg) reduced RBF and CBF, but increased MBF. BQ788 potentiated ET-1 mediated reductions in CBF, and abolished increases in MBF. BQ610 blunted reductions in RBF and CBF produced by ET-1, but did not significantly affect MBF responses. The renal vascular effects of intravenous ET-1 were mimicked by lower doses (1-30 ng/kg) administered into the renal artery.

CONCLUSIONS

Endogenous ETs act at ET(A)-receptors to reduce MBF and CBF, but ET(B)-receptors have little direct role in physiological control of renal haemodynamics. Bolus doses of ET-1 act at ET(B)-receptors in the kidney to increase MBF. The effects of bolus ET-1 on the cortical vasculature appear to result from the competing influences of ET(A)-mediated vasoconstriction and ET(B)-mediated vasodilatation.