Donald D. Smyth

Renal imidazoline preferring sites and solute excretion in the rat

1 Moxonidine has been found to have an approximately 600 fold greater affinity for I1 imidazoline preferring sites as compared to α2‐adrenoceptors in the rat kidney. The effects of an intrarenal infusion of moxonidine in an anaesthetized rat preparation were investigated and contrasted with the effects previously reported for α2‐adrenoceptor stimulation. 2 An intrarenal infusion of moxonidine (1, 3 and 10 nmol kg−1 min−1) produced an increase in urine flow rate and sodium excretion. Moxonidine increased urine volume through an increase in osmolar clearance rather than an increase in free water clearance as previously reported for α2‐adrenoceptor stimulation. 3 The effects of moxonidine also appeared to be unique from the effects of α2‐adrenoceptor stimulation. An imidazoline preferring site specific blocking dose of idazoxan (0.3 mg kg−1), but not an α2‐adrenoceptor specific blocking dose of rauwolscine (0.3 mg kg−1) attenuated the renal effects of moxonidine (3 nmol kg−1 min−1). Moreover, unlike α2‐adrenoceptor agonists, the effects of moxonidine were not altered by prior treatment with a V2 vasopressin receptor antagonist. 4 These results indicate differences between stimulation of α2‐adrenoceptors and I1 imidazoline preferring sites in the rat kidney and suggest a direct physiological function of renal imidazoline preferring sites.

Gender and age as factors in the inhibition of renal clearance of amantadine by quinine and quinidine

We studied the short‐term effect of oral doses of quinine and quinidine on the renal clearance of amantadine in healthy young (age range, 27 to 39 years) and older (age range, 60 to 72 years) adults of both genders in a three‐limbed randomized crossover study. Renal clearance of amantadine (13.2 ± 5.8 L/hr) was significantly inhibited by quinine (9.7 ± 4.8 L/hr) and quinidine (8.9 ± 4.0 L/hr) only in male subjects and was not associated with age. The chiral selectivity for the renal clearance of quinidine over quinine was confirmed and extended with the suggestion of both age‐ and gender‐associated changes on the renal clearance ratio for these two diastereomeric drugs. These data support the continued use of amantadine for studies on the renal elimination of organic cationic drugs.

Sodium excretion following central administration of an I1 imidazoline receptor agonist, moxonidine

1 Previously we have shown that an intrarenal infusion of moxonidine, an I1‐imidazoline receptor agonist, resulted in a natriuresis which was inhibited by intravenous idazoxan, a selective imidazoline receptor antagonist. Therefore we examined the effects on renal function of intracerebroventricular (i.c.v.) administration of moxonidine with or without i.c.v. idazoxan. 2 Seven days after unilateral nephrectomy, Sprague‐Dawley rats had i.c.v. cannulae implanted. Three days later the rats were anaesthetized (pentobarbitone), followed by cannulation of the jugular vein (fluid and drug administration), carotid artery (blood pressure) and the ureter (urine collection). 3 After a 45 min stabilization period, the effect of moxonidine was investigated by the i.c.v. administration of either isotonic saline or moxonidine (0.1, 0.3 or 1 nmol in isotonic saline) administered in 5 μl over 1 min. All doses of moxonidine resulted in an increase in urine flow with a concomitant increase in sodium excretion without affecting blood pressure. The highest dose of moxonidine (1 nmol) also increased free water clearance. 4 In a second series of experiments, the effects of idazoxan on the natriuretic response to i.c.v. moxonidine were determined. Moxonidine (0.3 nmol) again increased sodium and water excretion as compared to the i.c.v. saline control animals. Pretreatment with i.c.v. idazoxan (0.3 nmol), at a dose which alone failed to alter sodium and water excretion, completely attenuated the renal response to moxonidine. These results are consistent with central I1‐imidazoline receptors mediating a moxonidine‐induced increase in sodium and water excretion at doses that do not alter blood pressure.

Disparate effects of neuropeptide Y and clonidine on the excretion of sodium and water in the rat

Previous studies have demonstrated a similarity between the ability of neuropeptide Y (NPY) and clonidine to inhibit renin release and inhibit cAMP production. We therefore compared the effects of clonidine and NPY on the excretion of sodium and water in anesthetized rats which were unilaterally nephrectomized (right kidney) 10 days prior to the experiment. On the experimental day, rats were anesthetized (nembutal) and the left kidney exposed for the intrarenal infusion of the study drugs. The lowest dose of NPY (0.3 microgram/kg per min) investigated failed to alter renal function. Clonidine (0.3 microgram/kg per min) and NPY (1 microgram/kg per min) produced a similar increase in urine volume. Only NPY increased sodium excretion and osmolar clearance. Free water clearance was only increased by clonidine. Blood pressure and creatinine clearance were similar in all groups investigated. These effects were attenuated by pretreatment with pertussis toxin (5 days). The ability of pertussis toxin to block these effects suggests that the renal effects of NPY and clonidine are coupled to a G protein, conceivably the inhibitory Gi protein of the adenylate cyclase system. The disparate effects on sodium excretion and on free water and osmolar clearance indicate that the effects of these compounds may be mediated through the inhibition of different pools of hormonally stimulated cAMP.

Attenuated renal response to moxonidine and rilmenidine in one kidney-one clip hypertensive rats

1 I1 non‐adrenoceptor, imidazoline receptor agonists, such as moxonidine, increase urine flow rate and sodium excretion following infusion into the renal artery. The functions of these agonists in genetic and acquired models of hypertension have not been determined. 2 We therefore studied the renal effects of two known non‐adrenoceptor, imidazoline receptor agonists, rilmenidine and moxonidine, in 1K‐1C hypertensive and 1K‐sham normotensive rats. Rilmenidine (0, 3, 10, 30 nmol kg−1 min−1) or moxonidine (0, 1, 3, 10 nmol kg−1 min−1) was infused directly into the renal artery (30 gauge needle) of 1K‐sham normotensive and 1K‐1C hypertensive rats. 3 In 1K‐sham normotensive rats, rilmenidine and moxonidine produced dose related increases in urine flow rate, sodium excretion and osmolar clearance. Both rilmenidine and moxonidine failed to increase urine flow rate, sodium excretion and osmolar clearance in 1K‐1C hypertensive rats to the same extent as in 1K‐sham animals. At comparable doses, rilmenidine had no effect, while moxonidine (3 and 10 nmol kg−1 min−1) did result in a small increase in urine volume and osmolar clearance which was less than that observed in the 1K sham control animals. 4 These studies indicate that the renal effects of non‐adrenoceptor, imidazoline receptor stimulation are diminished in 1K‐1C hypertensive rats compared with 1K‐sham normotensive rats. Whether this decrease in activity of the natriuretic non‐adrenoceptor, imidazoline receptors contributes to the increase in blood pressure in the 1K‐1C acquired model of hypertension remains to be determined.

Effects of the selective I1 imidazoline receptor agonist, moxonidine, on gastric secretion and gastric mucosal injury in rats

1 Previous reports of the effects of α2‐adrenoceptor stimulation on gastric secretion are inconsistent because it was not clear whether the compounds were activating α2‐adrenoceptors and/or newly described imidazoline receptors. In the present experiments, the effects of moxonidine, an I1‐imidazoline receptor agonist and antihypertensive agent, on gastric secretion and on experimental gastric mucosal injury were examined. 2 Moxonidine (0.01, 0.1 and 1.0 mg kg−1, i.p.) potently inhibited basal (non‐stimulated) gastric acid secretion in conscious rats with an ED50 of 0.04 mg kg−1. Two hours following administration of the highest dose of moxonidine (1.0 mg kg−1), gastric acid output was completely suppressed. Moxonidine also significantly increased intragastric pH, at the two highest doses. 3 The α2‐adrenoceptor agonist, clonidine (0.01, 0.1 and 1.0 mg kg−1, i.p.) decreased basal acid secretion at the lowest dose (37%) and at the highest dose (46%), while the intermediate dose did not affect gastric acid output. 4 In an ethanol‐induced model of gastric mucosal injury, moxonidine decreased the length of lesions at the lowest and highest doses (0.01 and 1.0 mg kg−1) as well as the number of the lesions, at the highest dose (1.0 mg kg−1). 5 In pylorus‐ligated rats, moxonidine significantly decreased acid secretion (all doses), total secretory volume (1.0 mg kg−1) as well as pepsin output (1.0 mg kg−1). 6 In comparison to clonidine, moxonidine appears to be a more potent anti‐secretory and gastric‐protective compound. These data indicate a potential role for imidazoline receptor agonists in the management of gastroduodenal diseases associated with hypertension. The relative contribution of the central and peripheral effects of moxonidine to these gastrointestinal actions remains to be determined.

Clonidine‐induced increase in osmolar clearance and free water clearance via activation of two distinct α2‐adrenoceptor sites

1 Clonidine, an α2‐adrenoceptor agonist, will increase urine flow rate in the anaesthetized rat by increasing both free water and osmolar clearance. In the present study, we investigated whether these effects of clonidine were mediated at two sites which could be distinguished pharmacologically in uninephrectomized male Sprague‐Dawley rats. 2 Clonidine (1.0 nmol kg−1 min−1) infused into the renal artery increased osmolar and free water clearance. Following pretreatment with prazosin (0.15 mg kg−1, i.v.), an antagonist with reported selectivity for the α2b‐adrenoceptor subtype, the increase in free water but not osmolar clearance was decreased. Pretreatment with the opioid receptor antagonist, naltrexone (3.0 mg kg−1, i.v.) attenuated the increase in osmolar but not free water clearance. This disparate antagonism of clonidine by prazosin and naltrexone was consistent with two distinct sites. 3 We submit the hypothesis that the α2a‐ and α2b‐adrenoceptor subtypes mediated the clonidine‐induced osmolar and free water clearance. The blockade in free water clearance by prazosin indicated a possible role of the α2b‐adrenoceptor subtype whereas the α2a‐adrenoceptor subtype was considered as the site mediating the clonidine‐induced increase in osmolar clearance. UK‐14,304 (1.0 nmol kg−1 min−1), a mixed α2‐adrenoceptor/imidazoline receptor agonist with selectivity for the α2a‐subtype, increased only osmolar clearance. This increase was blocked by naltrexone but not prazosin pretreatment. The imidazoline receptor was not involved, as naltrexone failed to alter the moxonidine (3.0 nmol kg−1‐min−1) induced increase in osmolar clearance. These data suggested to us that the α2a‐/α2b‐subtype hypothesis should be investigated more closely in future studies. 4 These findings indicate that the increase in osmolar and free water clearance following clonidine can be distinguished pharmacologically indicating that two sites were involved. Furthermore, we propose the hypothesis that the α2a‐adrenoceptor subtype mediated osmolar clearance whereas the α2b‐subtype mediated free water clearance. The prazosin‐sensitive increase in free water clearance following clonidine suggested a possible role for the α2b‐subtype. The naltrexone‐sensitive increase in osmolar clearance following clonidine and UK‐14,304 (but not moxonidine) suggested a possible role of the α2a‐subtype. Clearly, this postulate requires further study.

A tea/vanadate decoction delivered orally over 14 months to diabetic rats induces long-term glycemic stability without organ toxicity

Vanadium can induce potent hypoglycemic effects in type 1 and type 2 diabetes mellitus animals, but toxic adverse effects have inhibited the translation of these findings. Administration of vanadate in a black tea decoction has shown impressive hypoglycemic effects without evidence of toxicity in short-term studies. The purpose of this study was to investigate the hypoglycemic action and the toxic adverse effects of a tea/vanadate (T/V) decoction in diabetic rats over a 14-month treatment period. Streptozotocin-induced type 1 diabetes mellitus rats were orally gavaged with 40 mg sodium vanadate in a black tea decoction only when blood glucose levels were greater than 10 mmol/L. Glycemic status and liver and kidney function were monitored over 14 months. All of the diabetic rats in this treatment group (n = 25) required treatment with the T/V decoction at the start of the study to reduce blood glucose levels to less than 10 mmol/L. Diarrhea was uncommon among the T/V-treated animals during the first week of T/V treatment and was absent thereafter. There was no evidence of liver or kidney dysfunction or injury. From 2 to 6 months, fewer animals required the T/V treatment to maintain their blood glucose levels. After 9 months of treatment, none of the diabetic animals required any T/V to maintain their blood glucose levels at less than 10 mmol/L. Oral administration of a T/V decoction provides safe, long-acting hypoglycemic effects in type 1 diabetes mellitus rats. The typical glycemic signs of diabetes were absent for the last 5 months of the study.

Natriuresis following Central and Peripheral Administration of Agmatine in the Rat

Agonists specific for the I1 imidazoline receptor increase sodium excretion following intrarenal (ir) infusion or intracerebroventricular (icv) injection in the rat. Although agmatine has been suggested to be a putative endogenous agonist for these receptors, the ability of this compound to alter sodium excretion has not been determined. The effects of agmatine, whether administered ir or icv, on blood pressure and solute and water excretion were studied in Sprague-Dawley rats. Agmatine was administered by icv injection (0, 10, 100, 300 or 1,000 nmol in 5 microliters) or by direct ir infusion (0, 3, 10, 30 or 100 nmol/kg/min at 3.4 microliters/min) in pentobarbitone-anesthetized rats. Agmatine administered by icv injection or ir infusion did not alter blood pressure or heart rate. Only an ir infusion of agmatine produced an increase in creatinine clearance, which occurred at the lowest (3 nmol/kg/min) and highest dose (100 nmol/kg/min). Concomitantly, the ir infusion of agmatine produced a dose-related increase in urine flow rate, but both routes of administration were associated with an increase in sodium excretion and osmolar clearance. Similar to previous reports with I1 imidazoline receptor-selective compounds, agmatine increased urine flow rate secondary to an increase in osmolar clearance at doses that failed to alter blood pressure. These results were consistent with agmatine functioning as a physiological agonist resulting in alterations in sodium excretion.

The role of the peripheral sympathetic nervous system in the natriuresis following central administration of an I1 imidazoline agonist, moxonidine.

1 . Central administration of the I1‐imidazoline receptor agonist moxonidine increases sodium excretion without alteration of blood pressure. In the present study we determined whether this natriuretic action was mediated through a decrease in activity of the sympathetic nervous system, as has been reported for the antihypertensive action of this compound. Interruption of the sympathetic nervous system was achieved with prazosin (α‐adrenoceptor antagonist) and renal denervation. 2 . In pentobarbitone‐anaesthetized Sprague‐Dawley rats, intracerebroventricular (i.c.v.) injection of moxonidine alone increased urine volume and sodium excretion. Prazosin (0.15 mg kg−1, i.v.) alone decreased urine flow rate and sodium excretion as compared to the vehicle controls. In the presence of prazosin, i.c.v. injection of moxonidine failed to increase sodium excretion or urine volume as compared to animals which received the prazosin alone. 3 . The administration of moxonidine (i.c.v.) to sham renal‐denervated animals caused an increase in urine flow rate, urine sodium excretion, osmolar clearance and free water clearance. The increase in sodium excretion and osmolar clearance were completely attenuated in renal denervated rats, however, urine flow rate was still increased and this was secondary to the increase in free water clearance which remained intact. 4 . These results indicate the importance of an intact sympathetic nervous system in the renal response to i.c.v. moxonidine. Moreover, the differential antagonism of these interventions on solute and water excretion indicate that they may be mediated at two separate sites and/or receptors following i.c.v. moxonidine.