Effects of streptozotocin-induced diabetes and insulin on calcium responsiveness of the rat vas deferens

In the present study, effects of streptozotocin-induced diabetes and insulin treatment on the reactivity of rat vas deferens to KCl and calmidazolium, a calmodulin antagonist, were evaluated and calmodulin levels in vas deferens tissue from diabetic and insulin-treated rats were determined. Diabetes was induced in rats by a single injection of streptozotocin. Five weeks after the induction of diabetes, one group of diabetic rats was injected with insulin for 3 weeks. After 8 weeks, vas deferens tissues on one side of diabetic and insulin-treated diabetic rats and their controls were mounted in organ bath to measure isometric tension, while the tissues on the other side of rats were homogenized to determine calmodulin levels by radioimmunoassay. Concentration-response curves to KCl were obtained in vas deferens tissues in the absence and presence of calmidazolium. The effects of KCl and calmidazolium on vas deferens isolated from 8-weeks diabetic rats were decreased. Calmodulin levels were also found to be decreased in vas deferens from diabetic rats. Decreased calmodulin levels in diabetic rat vas deferens were not corrected by insulin treatment. Only a partial correction following insulin treatment was observed in contractile effect of KCl on diabetic rat vas deferens, whereas insulin treatment increases the affinity of calmodulin in this muscle. Experimental diabetes causes an impairment in calcium/calmodulin-dependent contractile process of vas deferens, which is correctable partially following insulin therapy. The changes in the function of rat vas deferens due to streptozotocin diabetes seem to be related to impaired sexual functions in human diabetes.

Diabetes-induced decrease in rat brain microsomal Ca2+-ATPase activity

The Ca(2+)-ATPase activity of rat brain microsomes was studied in streptozotocin (STZ)-induced diabetes. Male rats, 200-250 g, were rendered diabetic by injection of STZ (45 mg kg(-1) body weight) via the teil vein. Brain tissues were collected at 1, 4 and 10 weeks after diabetes was induced for determination of Ca(2+)-ATPase activity, lipid peroxidation and tissue calcium levels. Diabetic rats had significantly elevated blood glucose levels compared to controls. Blood glucose levels were 92.92 +/- 1.22 mg dl(-1) (mean +/- SEM) for the control group, 362.50 +/- 9.61 mg dl(-1) at 1 week and >500 mg dl(-1) at 4, 8 and 10 weeks for the diabetics. Enzyme activities were significantly decreased at 1, 4, 8 and 10 weeks of diabetes relative to the control group (p < 0.001). Ca(2+)-ATPase activity was 0.084 +/- 0.008 U l(-1), 0.029 +/- 0.005 U l(-1), 0.029 +/- 0.006 U l(-1), 0.033 +/- 0.003 U l(-1) and 0.058 +/- 0.006 U l(-1) (mean +/- SEM) at control, 1, 4, 8 and 10 week of diabetes respectively. The change in calcium levels in diabetic rat brain at 8 and 10 weeks of diabetes was significantly higher than that of the control group (p < 0.05). On the other hand lipid peroxidation measured as TBARS (thiobarbituric acid reactive substances) was significantly higher at 8 and 10 weeks of diabetes (p < 0.05). The increase in lipid peroxidation observed in diabetic rat brain may be partly responsible for the decrease in calcium ATPase activity.

Glycation decreases calmodulin binding to lens transmembrane protein, MIP

Channels of the major intrinsic protein (MIP) of the lens transport water, thus playing an important role in lens fiber cell homeostasis. Calmodulin (CAM) interacts with MIP and possibly regulates MIP channel permeability. Protein glycation has been implicated in lens opacification. We previously identified sites of glycation of MIP, which are in close proximity to the putative CAM binding site. This study is aimed to show the effect of in vitro and in vivo glycation on CAM binding to MIP. Our results show that MIP and MP20 are the major CAM binding proteins of the lens membrane. In vitro incubation of lens membranes with 1 M glucose decreased CAM binding by 38% (P<0.001). Similarly, there was a progressive decrease in CAM binding to diabetic lens membranes compared to age-matched controls (up to 30% decrease, P<0.01). Mutation of K228 and K238 as well as a triple K mutation (K228N, K238N, K259N) of MIP resulted in a decrease in CAM binding. Thus, post-translational protein modifications of MIP influence CAM binding. Since CAM is the ubiquitous Ca(2+) receptor, decreases in CAM binding to the target protein will affect the Ca(2+)-mediated cellular processes leading to lens opacification in diabetic and aging lenses.

Effect of insulin treatment on smooth muscle contractility and endothelium-dependent relaxation in rat aortae from established STZ-induced diabetes

1. This study involved the chronic administration of low or high insulin to rats with established streptozotocin (STZ)-induced diabetes. We studied the effect of such treatment on smooth muscle contractility and endothelium-dependent relaxation using aortic strips. 2. Aortae from diabetic rats, but not those from high-insulin-treated diabetic rats, showed an impaired endothelium-dependent in response to acetylcholine (ACh) by comparison with untreated controls. 3. Isotonic high K+-induced contractility was impaired in diabetic aortae. This impairment was prevented by high-insulin treatment. 4. Noradrenaline (NA)-induced contractility was enhanced in aortae from high-insulin-treated diabetic rats, but not in those from untreated diabetic or low-insulin treated diabetic rats. 5. In the combined presence of the nitric oxide inhibitor N(G)-nitro-L-arginine and the cyclo-oxygenase inhibitor indomethacin, NA-induced contractility was significantly greater in aortae from high-insulin-treated diabetic rats than in those from controls or untreated diabetic rats. 6. An increased expression of the mRNA for the alpha1D and alpha1B adrenergic receptors was found in aortae from high-insulin-treated diabetic rats. 7. These results demonstrate that in rats with established STZ-induced diabetes, high-insulin treatment prevents the development of an impaired endothelium-dependent relaxation in the aorta, and that such treatment enhances NA-induced contractility. This enhancement may be related to an upregulation in the expression of the mRNA for the alpha1B or alpha1D adrenergic receptor that is secondary to the hyperinsulinaemia.

Changes in electrophysiological properties and noradrenaline response in vas deferens of diabetic rats

The purpose of this study was to study changes in the sympathetic nerves of the vas deferens in 10-week-old streptozotocin-induced diabetic rats. To assess the activity of autonomic neurons, we recorded the amplitude and frequency of spontaneous junction potentials in vasa deferentia from age-matched controls and streptozotocin-induced diabetic rats. No change in the resting membrane potential of the smooth muscle of the vas deferens was found in streptozotocin-induced diabetic rats. The frequency of spontaneous junction potentials was significantly increased in the streptozotocin-induced diabetic rats and their amplitude was also markedly increased. The dose-response curve for the contractile response of the vas deferens to noradrenaline was significantly shifted to the right and the apparent affinity (pD2 value) was significantly decreased in streptozotocin-induced diabetic rats. These results suggest that degeneration of sympathetic neurons may occur in the vas deferens of 10-week streptozotocin-induced diabetic rats and that the greater amplitude of the spontaneous junction potentials may be related to an increase in Ca2+ mobilization, though the increase in Ca2+ mobilization does not lead to an enhanced contractile response.