Calcium channel agonist/antagonist effects on cholinergic stimulation of the diabetic rat bladder

Renal actions of the α2-adrenoceptor agonist, xylazine, in the anaesthetised rat

AIMS

The aims of the present study were to characterise the renal effects of the alpha2-adrenergic agonist, xylazine, in the rat and to test the role of changes in glomerular filtration rate, glucosuria, and arginine vasopressin (AVP) in its mechanism of action.

METHODS

Male Wistar rats were anaesthetised with pentobarbitone sodium (50 mg/kg), and polyethylene cannulae were surgically placed for blood pressure measurement and for blood and urine collection. Rats were given xylazine and other alpha2 agonists by bolus intravenous dose, and the effects of the drugs were monitored in the presence and absence of the selective alpha2 antagonist, yohimbine, the alpha1, alpha2B antagonist, prazosin, and the V2-receptor antagonist, d(CH2)5 [D-Ile2,Ile4,Ala-NH29]AVP.

RESULTS

Xylazine at 2.5 mg/kg caused a significant and prolonged dose-dependent increase in urine flow rate and sodium excretion but had only short-lasting effects on blood pressure, heart rate, and glomerular filtration rate. Prazosin had no effect on the measured responses. Although plasma glucose concentration and glucose excretion rate were increased by xylazine, the magnitudes of these increases were insufficient to account for the diuresis observed. Xylazine, and 2 other alpha2 agonists, clonidine and oxymetazoline, increased urine flow and/or sodium excretion despite the presence of d(CH2)5 [D-Ile2,Ile4,Ala-NH29]AVP.

CONCLUSIONS

Xylazine causes a diuretic and natriuretic alpha2A-adrenergic response in the rat that is independent of changes in glomerular filtration rate, the development of glucosuria, or AVP action on the distal nephron of the kidney.

CLINICAL RELEVANCE

The adverse effects of xylazine on salt and water balance need to be considered and possibly compensated for by fluid replacement or post-surgical administration of alpha2-receptor antagonists.

Smooth muscle of the bladder in the normal and the diseased state: pathophysiology, diagnosis and treatment

The smooth muscle of the normal bladder wall must have some specific properties. It must be very compliant and able to reorganise itself during filling and emptying to accommodate the change in volume without generating any intravesical pressure, but whilst maintaining the normal shape of the bladder. It must be capable of synchronous activation to generate intravesical pressure at any length to allow voiding. The cells achieve this through spontaneous electrical activity combined with poor electrical coupling between cells, and a dense excitatory innervation. In the diseased state, alterations of the smooth muscle may lead to failure to store or failure to empty properly. The diseased states discussed are bladder instability and diabetic neuropathy. Bladder instability is characterised urodynamically by uninhibitable rises in pressure during filling, and is seen idiopathically and in association with bladder outflow obstruction and neuropathy. In diabetic neuropathy, many of the smooth muscle changes are a consequence of diuresis, but there is evidence for alterations in the sensory arm of the micturition reflex. In the unstable bladder, additional alterations of the smooth muscle are seen, which are probably caused by the patchy denervation that occurs. The causes of this denervation are not fully established. Nonsurgical treatment of instability is not yet satisfactory; neuromodulation has some promise, but is expensive, and the mechanisms poorly understood. Pharmacological treatment is largely through muscarinic receptor blockade. Drugs to reduce the excitability of the smooth muscle are being sought, since they may represent a better pharmacological option.

Factors underlying the increased sensitivity to field stimulation of urinary bladder strips from streptozotocin-induced diabetic rats

1. The responses of bladder strips from control, streptozotocin-diabetic, and sucrose-drinking rats to electrical field stimulation were investigated. Sucrose-drinking rats were included as additional controls because they have enlarged bladders as a result of non-diabetic diuresis. 2. Bladder strips from diabetic rats developed more spontaneous activity than those from the two control groups. Indomethacin reduced the amplitude and frequency of spontaneous contractions suggesting that they resulted from endogenous prostaglandin formation. Tetrodotoxin (TTX) had little effect, while alpha, beta-methylene ATP caused increases in spontaneous activity. 3. Bladder strips from diabetic rats responded to field stimulation with greater contractions than controls in the absence of antagonists as well as in the presence of atropine and alpha, beta-methylene ATP. Increasing TTX concentrations caused a step-wise depression of the contractile response to electrical stimulation which was not affected by preincubation with either atropine or alpha, beta-methylene ATP. 4. Atropine and indomethacin had no effect on strength-duration curves constructed to measure threshold contractile responses to five pulses stimulation. The curves were shifted to the right by both TTX and alpha, beta-methylene ATP, indicating that the responses were neurogenic in nature and at least partially, the result of stimulation of P2-purinoceptors. In the absence of drugs, bladder strips from diabetics responded at lower voltages and pulse widths than those of control and sucrose-drinking rats, suggesting that they were more excitable. 5. The response curve of bladder strips from diabetics to field stimulation at increasing voltage was shifted upwards and to the left compared to strips from control or sucrose-drinking rats. 6. Bladder strips from diabetics responded to stimulation at increasing pulse width with greater responses than those from control or sucrose-drinking rats. At 1.0 ms pulse width, the TTX-resistant response of strips from diabetic rats was still greater than that of the other groups, indicating that a myogenic component was also involved.7. The data suggest that bladder strips from diabetic rats are more excitable than those of control or sucrose-drinking rats. This may result from diabetes-induced decreases in bladder lipid or other membrane changes, and/or be a result of partial depolarization, perhaps related to diabetic neuropathy.

Urinary bladder dysfunction in the BB/W diabetic rat: effect of ganglioside treatment on functional and structural alterations

Urinary bladder dysfunction in the diabetic BB/W rat is characterized by infrequent irregular contractions of high amplitude. Initially these occur in the absence of detectable neuroanatomical lesions of sensory afferent and parasympathetic fibers of the pelvic nerve, which constitute the micturition reflex arc. Structural lesions consisting of progressive axonal atrophy of myelinated and unmyelinated fibers become detectable only after 4 months of diabetes. In the current study we evaluated the effect of ganglioside treatment (10 mg./kg. body weight) for one month. This drug regimen was initiated at 4 months of diabetes, when functional bladder abnormalities were well established, whereas structural lesions were yet to appear. Animals examined 1 or 3 months after termination of the one-month treatment protocol showed sustained normalization of the characteristic functional abnormalities, accompanied by prevention of the neuroanatomical lesions of sensory afferent and parasympathetic efferent myelinated fibers in the pelvic nerve. These data suggest that ganglioside treatment may be beneficial in delaying the progression of diabetic autonomic neuropathy in this experimental animal model.

Synthesis and release of acetylcholine in the rabbit kidney cortex

Several cholinergic processes were demonstrated and partially characterized in rabbit kidney cortical minces: choline uptake, acetylcholine synthesis and calcium-dependent release. Minces took up labelled choline, acetylated it, and stored it in a pool that was not readily accessible to physostigmine-sensitive cholinesterase activity. [3H]Acetylcholine synthesis but not [3H]choline uptake was inhibited by the removal of sodium ions or incubation at 0 degrees C. The release of newly synthesized [3H]acetylcholine was increased by 300 mOsmol urea in a calcium-dependent manner, but not by potassium depolarization (300 mOsmol), vasopressin (10 microM), or bradykinin (10 microM). These results suggest that acetylcholine may be synthesized by non-neuronal rabbit kidney cortical cells and that this transmitter may be released in response to physiological levels of urea.