Inhibitory effect of streptozotocin-induced diabetes on non-cholinergic motor transmission in rat detrusor and its prevention by sorbinil

Purinergic signalling in the urinary tract in health and disease

Purinergic signalling is involved in a number of physiological and pathophysiological activities in the lower urinary tract. In the bladder of laboratory animals there is parasympathetic excitatory cotransmission with the purinergic and cholinergic components being approximately equal, acting via P2X1 and muscarinic receptors, respectively. Purinergic mechanosensory transduction occurs where ATP, released from urothelial cells during distension of bladder and ureter, acts on P2X3 and P2X2/3 receptors on suburothelial sensory nerves to initiate the voiding reflex, via low threshold fibres, and nociception, via high threshold fibres. In human bladder the purinergic component of parasympathetic cotransmission is less than 3 %, but in pathological conditions, such as interstitial cystitis, obstructed and neuropathic bladder, the purinergic component is increased to 40 %. Other pathological conditions of the bladder have been shown to involve purinoceptor-mediated activities, including multiple sclerosis, ischaemia, diabetes, cancer and bacterial infections. In the ureter, P2X7 receptors have been implicated in inflammation and fibrosis. Purinergic therapeutic strategies are being explored that hopefully will be developed and bring benefit and relief to many patients with urinary tract disorders.

Purinergic signalling and diabetes

The pancreas is an organ with a central role in nutrient breakdown, nutrient sensing and release of hormones regulating whole body nutrient homeostasis. In diabetes mellitus, the balance is broken-cells can be starving in the midst of plenty. There are indications that the incidence of diabetes type 1 and 2, and possibly pancreatogenic diabetes, is rising globally. Events leading to insulin secretion and action are complex, but there is emerging evidence that intracellular nucleotides and nucleotides are not only important as intracellular energy molecules but also as extracellular signalling molecules in purinergic signalling cascades. This signalling takes place at the level of the pancreas, where the close apposition of various cells-endocrine, exocrine, stromal and immune cells-contributes to the integrated function. Following an introduction to diabetes, the pancreas and purinergic signalling, we will focus on the role of purinergic signalling and its changes associated with diabetes in the pancreas and selected tissues/organ systems affected by hyperglycaemia and other stress molecules of diabetes. Since this is the first review of this kind, a comprehensive historical angle is taken, and common and divergent roles of receptors for nucleotides and nucleosides in different organ systems will be given. This integrated picture will aid our understanding of the challenges of the potential and currently used drugs targeted to specific organ/cells or disorders associated with diabetes.

Transcutaneous electrical nerve stimulation (TENS) improves the diabetic cytopathy (DCP) via up-regulation of CGRP and cAMP

The objective of this study was to investigate the effects and mechanism of Transcutaneous Electrical Nerve Stimulation (TENS) on the diabetic cytopathy (DCP) in the diabetic bladder. A total of 45 rats were randomly divided into diabetes mellitus (DM)/TENS group (n = 15), DM group (n = 15) and control group (n = 15). The rats in the DM/TENS and TENS groups were electronically stimulated (stimulating parameters: intensity-31 V, frequency-31 Hz, and duration of stimulation of 15 min) for three weeks. Bladder histology, urodynamics and contractile responses to field stimulation and carbachol were determined. The expression of calcitonin gene-related peptide (CGRP) was analyzed by RT-PCR and Western blotting. The results showed that contractile responses of the DM rats were ameliorated after 3 weeks of TENS. Furthermore, TENS significantly increased bladder wet weight, volume threshold for micturition and reduced PVR, V% and cAMP content of the bladder. The mRNA and protein levels of CGRP in dorsal root ganglion (DRG) in the DM/TENS group were higher than those in the DM group. TENS also significantly up-regulated the cAMP content in the bladder body and base compared with diabetic rats. We conclude that TENS can significantly improve the urine contractility and ameliorate the feeling of bladder fullness in DM rats possibly via up-regulation of cAMP and CGRP in DRG.

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.

The aldose reductase inhibitor sorbinil does not prevent the impairment in nitric oxide-mediated neurotransmission in anococcygeus muscle from diabetic rats

This study investigated whether increased polyol pathway activity could contribute to alterations in nitrergic neurotransmission in anococcygeus muscles from 8-week diabetic rats. In the presence of guanethidine (10-30 microM) and clonidine (0.01-0.05 microM), relaxations obtained to nitrergic nerve stimulation (0.5-5 Hz, 10-s train), to sodium nitroprusside (5-500 nM) and to nitric oxide (0.1-3 microM) were significantly reduced in muscles from diabetic rats compared to responses from control rats. Treatment of diabetic rats with the aldose reductase inhibitor sorbinil (42 mg/kg per day via feed for 8 weeks) did not affect impaired reactivity to nitrergic nerve stimulation, sodium nitroprusside or nitric oxide. The results suggest increased polyol pathway activity does not contribute to the alterations in nitrergic neurotransmission in anococcygeus muscles from diabetic rats.

Altered contractility of urinary bladder in diabetic rabbits: relationship to reduced Na+ pump activity

We studied the effect of alloxan-induced diabetes on Na+ pump activity in isolated rabbit bladder strips. In addition, the effects of diabetes and the Na+ pump inhibitor ouabain on contractions induced by carbachol (CCh) and KCl were studied. In bladder strips from diabetic rabbits, ouabain-sensitive 86Rb+ uptake (a measure of Na+ pump activity) was approximately 50% less compared with strips from normal bladder. Diabetes also reduced the maximum contractions induced by CCh and KCl. Treatment of bladder strips with ouabain alone caused an acute concentration-dependent increase in tone. In contrast, longer incubation with ouabain inhibited CCh- and KCl-induced contractions in normal and diabetic bladders. Furthermore, differences in agonist-mediated contractions observed between normal and diabetic bladders were abolished in the presence of the maximally effective concentration of ouabain (10 microM). The ability of CCh to cause contraction in normal and diabetic rabbit bladders was also significantly inhibited by the Na+ ionophore monensin but not by the Ca2+ ionophore A-23187 or by depolarization with KCl. Monensin also inhibited KCl-induced contractions in normal bladder strips. These results indicate that 1) Na+ pump activity is an important modulator of bladder smooth muscle tone, 2) diabetes diminishes Na+ pump activity and inhibits agonist-induced contractions in bladder, and 3) an increase in intracellular Na+ concentration, secondary to inhibition of bladder smooth muscle Na+ pump activity, is associated with reduced responsiveness to contractile agonists. Diminished Na+ pump activity in diabetes may, in part, contribute to the development of bladder cystopathy.

The effects of uranyl ions on neuromuscular transmission in the urinary bladder of the normal and streptozotocin-diabetic mouse

The depressant effects of uranyl nitrate on the nerve-evoked muscle contraction of urinary bladder isolated from normal and streptozotocin-diabetic mice were compared. The non-cholinergic component of the evoked bladder contraction (in the presence of atropine) was specifically sensitive to the suppressive effect of uranyl nitrate. In contrast, the cholinergic component remaining after treatment with alpha, beta-methylene ATP was rather insensitive to uranyl nitrate. The contractile responses induced by KCl, acetylcholine and ATP were also not affected by uranyl nitrate, indicating a presynaptic site of action. High Ca2+ and calmodulin inhibitors (trifluoperazine, diltiazem and W7) antagonized the suppressive effects of uranyl ions. These results suggest that the depressant effects of uranyl nitrate is mediated by a reduction of prejunctional non-cholinergic transmitter release through the calcium-calmodulin pathway. In contrast to the normal bladder, the urinary bladder of streptozotocin-diabetic mice revealed not only weaker neurogenic contractile responses to electrical field stimulation, but also a profound reduction in the depressant effect of uranyl nitrate. These findings suggest that the Ca2+ regulation of non-cholinergic neurotransmission in mouse urinary bladder may be impaired in the diabetic state.

Enhancement of muscarinic receptor-coupled phosphatidyl inositol hydrolysis in diabetic bladder

We previously have shown an increase in muscarinic receptor density in streptozotocin (STZ)-induced diabetic and sucrose-fed diuretic rat detrusor that correlates with an increase in the contractile response to muscarinic agonist (J Pharmacol Exp Ther 248:81, 1989; Diabetes 40: 265, 1991). To investigate the signal transduction pathway involved in this altered functional response, we examined muscarinic receptor-coupled phosphatidylinositol metabolism in STZ-diabetic, sucrose-fed diuretic and age-matched control rat bladders. [3H]myo-inositol uptake was similar in all groups, but incorporation of myo-inositol into phosphatidylinositol (PI) was significantly increased in the diabetic bladder compared to the sucrose-fed and control rat bladders. Carbachol-induced increase in inositol phosphate (IPs) production was higher in the diabetic bladder than in bladders from control and sucrose-fed animals although the EC50 values were similar for all groups. Enhanced inositol phosphate production after muscarinic agonist stimulation may be due not only to the upregulation of muscarinic receptors but also the increased incorporation of myo-inositol into PI in the STZ-induced diabetic bladder.

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.

Rat: overview and innervation

Despite the development of molecular and cellular methods for examining physiological processes, the use of the whole animal model remains essential to advance knowledge regarding the integration and coordination of events associated with urinary tract function. The rat offers an inexpensive and versatile species to investigate bladder and urethral responses to drugs or pathology. Models for many disorders have been developed in rodents including diabetes, multiple sclerosis, spinal cord injury, Parkinson's disease, bladder outlet obstruction, pain, and aging. This review examines methodologies to evaluate lower urinary tract function and manipulations used to create pathological models in rodents.

Changes in contractile responses of the urinary bladder to substance P in streptozotocin-induced diabetic rats

1. Functional changes in the urinary bladder obtained from streptozotocin-induced diabetic rats were investigated by determining the responsiveness of bladder strips to capsaicin or substance P (SP). 2. Contractile responses of detrusor strips of the urinary bladder in response to capsaicin were almost abolished in both diabetic rats and capsaicin-pretreated rats. 3. Maximal contractions of diabetic detrusor strips induced by SP were significantly increased when compared to age-matched controls. 4. In contrast to the contractile responses to SP, the density of SP receptors was significantly decreased in diabetic rats. 5. The increased contractile responses to SP were markedly decreased by treatment with indomethacin, OKY-046 or quinacrine, but not with nordihydroguaiaretic acid. 6. Contractile responses of detrusor strips to prostaglandin F2 alpha and E2 were unchanged in the diabetic state. 7. These results suggest that the increased contractile responses of detrusor strips of the bladder to SP in the diabetic state are due to increased synthesis of prostaglandins and/or thromboxane A2 via the increased activity of phospholipase A2 on the smooth muscle of the diabetic bladder.

The action of palytoxin on the isolated detrusor muscle of the rat

1. The effects of a coelenterate toxin, palytoxin (PTX) have been studied in the isolated detrusor muscle. of the rat. 2. PTX (1-100 nM) initiated concentration-dependent contractions of the detrusor; the contraction led to an irreversible tachyphylaxis. Muscle desensitized to PTX continued to respond to acetylcholine (ACh) and excess K+ but the contractions were reduced compared to pre-PTX contractions. 3. Contractions evoked by PTX were not affected by the presence of atropine (10 microM), indomethacin (10 microM) or tetrodotoxin (0.5 microM) but were greatly reduced by nifedipine (3 microM) and by the absence of K+. PTX could not evoke contractions in the absence of Ca2+ or in tissues depolarized by exposure to excess K+. 4. PTX abolished the neurogenic contractile responses to electrical field stimulation (EFS). 5. Combined treatment with atropine (10 microM) plus nifedipine (3 microM) abolished contractile responses to EFS and greatly reduced the contractile response to PTX. 6. The contractile response to PTX (100 nM) was reduced following exposure of the muscle to alpha, beta-methylene ATP. 7. Exposure to PTX (100 nM) for 1-3 h reduced both the ACh content of the detrusor (by more than 80%), and the immunoreactivity of neuropeptide Y-containing nerve fibres compared to control. 8. It is concluded that the primary effect of PTX is to promote the release of endogenous motor transmitters, leading to their eventual depletion.

Insulin secretion from pancreatic B cells caused by L-arginine-derived nitrogen oxides

L-arginine causes insulin release from pancreatic B cells. Data from three model systems support the hypothesis that L-arginine-derived nitrogen oxides (NOs) mediate insulin release stimulated by L-arginine in the presence of D-glucose and by the hypoglycemic drug tolbutamide. The formation of NO in pancreatic B cells was detected both chemically and by the NO-induced accumulation of guanosine 3',5'-monophosphate. NG-substituted L-arginine analogs inhibited the release of both insulin and NO. Protein immunoblot and histochemical analysis with antiserum to type I NO synthase suggest that the formation of NO in pancreatic B cells is catalyzed by an NADPH- (reduced form of nicotinamide adenine dinucleotide phosphate), Ca2+/calmodulin-dependent type I NO synthase of about 150 kilodaltons.

Aldose reductase inhibitors as pathobiochemical probes

In tissues susceptible to damage from chronic diabetes, excess glucose is metabolized by aldose reductase (AR) to sorbitol. Originally, AR-catalyzed sorbitol formation (and accumulation) was found in the diabetic lens; the cataractogenicity of this process was proven by preventing cataract formation with an AR inhibitor (ARI). These findings were extended to the hypothesis that, in diabetic tissues, excessive intracellular sorbitol formation initiates a cascade of metabolic abnormalities which gradually progress to loss of functional and structural integrity. The pivotal role of AR as a trigger for such abnormalities was established by preventing their occurrence in diabetic animals treated with an ARI. By inference, this led to the concept that inhibition of AR should prevent, arrest, and, possibly, reverse the development of late diabetic sequelae. In addition to motivating drug-oriented research, the ARI concept provided a rationale for the use of ARIs as experimental tools to probe the pathogenesis of diabetic complications. By helping to elucidate the metabolic, functional, and structural ramifications of the AR-catalyzed disposal of excess glucose in diabetic schemes, and in addition, by helping to define the applicability of animal models for the study of early functional pathogenic alterations occurring in diabetic subjects, ARIs may enable the discrimination in diabetic tissue of arrestible and reversible from the irreversible abnormalities.

The effect of streptozotocin-induced diabetes on cholinergic motor transmission in the rat urinary bladder

1. The effect of streptozotocin (STZ)-induced diabetes on cholinergic motor transmission in the rat urinary bladder was investigated by recording contractile activity of detrusor strips in vitro. 2. The Ca(2+)-channel antagonist, nifedipine, was found to be more effective in blocking the noncholinergic motor transmission than P2-purinoceptor desensitization by alpha,beta-methylene ATP. 3. The neurogenic contractile responses to electrical field stimulation in the presence of nifedipine (cholinergic) were larger in the diabetic detrusor than in the non-diabetic controls. The potentiation of the cholinergic transmission was more evident at higher frequencies. 4. Concentration-response curves for acetylcholine were identical in detrusors from diabetic and non-diabetic animals, thus excluding a postsynaptic supersensitivity to acetylcholine being responsible for the potentiation of cholinergic motor transmission. 5. It is concluded that the potentiation of cholinergic motor transmission is due to enhanced release of acetylcholine in diabetic detrusor. Possible reasons for this enhancement are discussed in relation to diabetes.