Kaushik P. Patel

Alterations in brain hexokinase activity associated with streptozoticin-induced diabetes mellitus in the rat

The effects of streptozotocin-induced diabetes mellitus on the activity of discrete regions of the brain were studied with histochemical localization and photodensitometric quantification of the metabolic enzyme, hexokinase. Two weeks after a single injection of streptozotocin (65 mg/kg, i.p.), plasma glucose and osmolarity levels were elevated, and plasma sodium concentrations were depressed. These changes were reversed in diabetic rats treated with insulin. Accompanying these symptoms of diabetes were significant increases in hexokinase activity in the magnocellular division of the paraventricular nucleus of the hypothalamus (mPVH, 12.1%), the medial subdivision of the nucleus of the tractus solitarius (mNTS, 15.5%), and the commissural subdivision of the NTS (cNTS, 10.9%). An increase, though just below the level of significance, was also observed in the supraoptic nucleus of the hypothalamus (SON, 11.5%). The increases in hexokinase activity were completely reversed in the cNTS (and SON) and only partly reversed in the mPVH and mNTS of insulin-treated diabetic rats. No changes in hexokinase activity were seen in the subfornical organ, medial preoptic area, parvocellular division of the PVH, locus coeruleus, or dorsal motor nucleus of the vagus of diabetic rats. These results reinforce the idea that the brain is not exempt from changes associated with diabetes mellitus and suggest that metabolic alterations in the mPVH (and SON) and two divisions of the NTS are likely related to changes in vasopressin production and blood volume, respectively.

Central alpha-2 adrenergic mechanisms in the renal nerve mediated natriuresis and diuresis produced by acute volume expansion

To determine whether central alpha-2 adrenergic mechanisms are involved in the renal nerve mediated natriuresis and diuresis produced by acute volume expansion, urine flow and sodium excretion from innervated and denervated kidneys were measured before and after acute volume expansion (1 ml/min for 20 min) in the presence or absence of intracerebroventricular yohimbine (8 micrograms/kg/min), an alpha-2-antagonist, in Inactin-anesthetized Sprague-Dawley rats. The innervated to denervated (I/D) ratio for urine flow and sodium excretion indicated that acute volume expansion caused a greater natriuresis and diuresis from the intact kidney compared to the denervated kidney. However, these I/D ratios during acute volume expansion were significantly reduced in the presence of yohimbine i.c.v. Furthermore, central administration of clonidine, an alpha-2 agonist, produces a renal nerve mediated natriuresis. These data suggest that central alpha-2 adrenergic mechanisms may be involved in producing the renal sympatho-inhibition, which subsequently produces natriuresis and diuresis, in response to acute volume expansion.

Renal responses to acute volume expansion and atrial natriuretic factor in streptozotocin-induced diabetic rats

The purpose of this study was to determine whether diuretic and natriuretic effects are altered in response to volume expansion (VE) and atrial natriuretic factor (ANF) in 4-week diabetic rats. Diabetes was induced in two groups of male Sprague Dawley rats using streptozotocin (STZ), while a control group of rats was treated with vehicle alone, four weeks prior to the experiment. One group of diabetic rats was treated daily with insulin for the four weeks prior to the experiment. Before, during and after VE (1.2 ml/min for 15 min), urine flow and sodium excretion were measured from innervated and denervated kidneys in the three groups of anesthetized rats. Then the renal response to infusion of ANF (0.25 microgram/kg/min for 15 min) were observed in these rats. During VE, urine flow and sodium excretion from innervated kidneys, but not from denervated kidneys, were significantly lower in diabetic rats than those in control rats. Urine flow and sodium excretion from innervated as well as denervated kidneys of the diabetic rats failed to increase compared to the control rats in response to ANF. Correcting the diabetic condition with insulin (third group) rectified the blood glucose levels and the blunted responses to either VE or ANF. At the initial level, glomerular filtration rate (GFR) was not significantly different among the three groups. During VE and ANF infusion, changes in GFR was not parallel to changes in excretory parameters, therefore the hemodynamic change may not be the main reason for the blunted renal responses in diabetic rats. This study demonstrates that: (1) the volume reflex is blunted in the 4-week diabetic rats, which is in part due to the presence of tonic renal nerve activity, (2) renal responses to ANF are blunted in the 4-week diabetic rats, and (3) insulin treatment in diabetic rats normalizes the altered renal responses to either acute volume expansion or ANF.

Renal Responses to Acute Volume Expansion in Young Spontaneously Hypertensive Rats

The renal responses to acute volume expansion (VE) were measured from intact and denervated kidneys in 5 week old spontaneously hypertensive rats (SHR). Urine flow and sodium excretion were measured before, during and after VE from innervated and denervated kidneys in anesthetized (Inactin--0.1 g/kg, ip) age and sex matched SHR and normotensive Wistar Kyoto (WKY) rats. Mean arterial pressure was 23 mm Hg higher in SHR than in WKY. During VE the increment in urinary flow rate and sodium excretion from both innervated and denervated kidneys were greater in SHR than WKY. In another group of SHR, renal perfusion pressure was maintained at a level similar to the arterial pressure in the WKY group (84 mm Hg). When renal perfusion pressure was controlled at the lower level in the SHR there was no longer the increase in diuresis and natriuresis in response to acute VE. Examining differences within a strain, the SHR demonstrate a greater increase in diuresis and natriuresis in response to VE in the absence of renal nerves, unlike the WKY. In conclusion, these results suggest that there is a greater diuresis and natriuresis in the young SHR due to increased renal perfusion pressure and the renal nerves produce a greater retention of water and sodium in SHR compared to WKY.

Blunted Diuretic and Natriuretic Responses to Central Administration of Clonidine in Streptozocin-Induced Diabetic Rats

The purpose of this study was to determine whether diuretic and natriuretic effects are altered in response to intracerebroventricular (ICV) infusion of clonidine in diabetic rats. Diabetes was induced in male Sprague-Dawley rats by 65 mg/kg i.p. injection of streptozocin, and control rats were injected with vehicle 2 wk before the experiment. Blood glucose levels were significantly elevated in the diabetic group (26.3 ± 1.3 mM) compared with the control group (8.4 ± 1.6 mM). Before and during ICV infusion of clonidine (2 μg · kg−1 · min−1 for 45 min), urine flow and sodium excretion were measured from intact and denervated kidneys in anesthetized diabetic and control rats. The ICV infusion of clonidine significantly increased urine flow in both innervated and denervated kidneys from control rats but not from diabetic rats. There was a significant increase in sodium excretion during ICV infusion of clonidine from innervated kidneys of control rats, and denervation abolished this effect. In diabetic rats, clonidine failed to promote natriuresis from intact kidneys, and similar to control rats, did not promote natriuresis in denervated kidneys. This study demonstrates that 1) the diuretic response to the ICV infusion of clonidine is blunted in diabetic rats, and 2) a natriuretic response to the ICV infusion of clonidine is blunted in innervated kidneys of diabetic rats.

Interaction among atrial natriuretic factor (ANF), vasopressin, and renal nerves in terms of renal responses in rats

The relationship between the renal nerves and vasopressin in terms of the natriuretic and diuretic responses to atrial natriuretic factor (ANF--0.25 microgram/kg/min for 15 min), was investigated in unilaterally denervated anesthetized rats before and after the administration of a vasopressin V2 specific antagonist (AVPX)--(40 micrograms/kg bolus followed by 0.4 microgram/kg/min infusion). Administration of the AVPX or ANF did not alter the arterial pressure. Acute renal denervation or AVPX administration independently produced significant increases in sodium and water excretion. ANF infusion by itself produced a greater increase in urine flow and sodium excretion from the denervated kidney compared to the intact kidney before the administration of AVPX. However, after the administration of AVPX renal responses to ANF from the intact kidneys were enhanced such that they were not significantly different from the denervated kidneys. These results suggest that the full physiological response to ANF may be masked by tonic renal nerve activity or antidiuretic actions of vasopressin. Furthermore, since combined renal denervation and AVPX administration does not produce any greater potentiation of the renal responses to ANF than either of these manipulations alone, it is suggested that they may act via a common mechanism, possibly altering activity in the renal nerves.

Diuretic and natriuretic responses to ANF in the presence and absence of renal nerves in DOCA-salt hypertensive rats.

To determine if renal nerves contributes in the renal response to atrial natriuretic factor (ANF) in DOCA-salt hypertensive rats, diuretic and natriuretic responses to ANF were measured in Inactin (0.1 g/kg, i.p) anesthetized rats with unilateral renal denervation. Rats were assigned to either a control group (108 +/- 6 mmHg), or one of two DOCA-salt groups (injected with deoxycorticosterone acetate, DOCA, 25 mg/week, and given 0.9% saline to drink for 4 weeks); a) DOCA-salt group (137 +/- 6 mmHg) and b) DOCA-salt-BPC group (with blood pressure controlled at the level of the femoral artery (102 +/- 3 mmHg) by an occluder on the abdominal aorta proximal to the right renal artery). Urine flow and sodium excretion in response to ANF infusion (0.3 micrograms/min/kg) were measured from intact and denervated kidneys of control and DOCA-salt treated rats. ANF infusion produced a significant increase in diuresis and natriuresis in all three groups of rats. Urine flow and sodium excretion in response to ANF were significantly less in the intact kidney but not the denervated kidneys of the DOCA-salt rats compared to control rats. These results indicate that renal nerves contribute to the blunted renal responses to ANF in DOCA-salt rats. Renal responses also were significantly smaller in both intact and denervated kidneys of DOCA-salt-BPC rats (in which arterial pressure was reduced) compared to DOCA-salt rats. Overall, these results indicate that both renal nerves and arterial pressure determine the natriuretic and diuretic actions of ANF in DOCA-salt hypertensive rats.

Role of renal nerves and vasopressin in renal responses to acute volume expansion in rats

Abstract This study was to determine whether the presence or absence of renal nerves and vasopressin altered the diuretic and natriuretic responses to acute volume expansion. Two forms of volume expansion were used: (i) inflation of a small balloon in the veno-atrial junction and (ii) an infusion of isotonic saline at a rate of 1 ml/min for a period of 15 min, approximately 7% of body weight. Balloon inflation produced a significant diuresis from both the intact and denervated kidneys but only produced a significant natriuresis from the intact kidney. Volume expansion (infusion of saline) produced a significant diuresis and natriuresis from both intact and denervated kidneys. Blocking the V2 receptor for vasopressin with a V2-specific receptor blocker d(CH2)5[olle2,Val4]AVP (40 μg/kg bolus dose followed by infusion of 4 μg/kg/min) did not alter the diuretic and natriuretic responses to volume expansion. However, the absence of renal nerves or the absence of actions of vasopressin produced a significant reduction in the capacity of the kidneys to increase the relative amount of diuresis or natriuresis, thus losing the control over output; i.e., absence of renal nerves only allowed 12-fold increase in diuresis to volume expansion compared with 25-fold in the intact state and absence of vasopressin only allowed 4.6-fold increase in diuresis to volume expansion compared with 25-fold in the intact state. Examining the “volume reflex” in terms of a control system trying to regulate fluid balance, the presence of either renal nerves or actions of vasopressin allows the volume regulating system a greater range in which to control the diuresis and natriuresis (making it possible to fine tune the output to much greater extent).