Gender differences in the effects of streptozotocin-induced diabetes on parasympathetic vasodilatation in the rat submandibular gland

17β-Estradiol Treatment Improves Acetylcholine-Induced Relaxation of Mesenteric Arteries in Ovariectomized UC Davis Type 2 Diabetes Mellitus Rats in Prediabetic State

We recently reported sex differences in mesenteric arterial function of the UC Davis type-2 diabetes mellitus (UCD-T2DM) rats as early as the prediabetic state. We reported that mesenteric arteries (MA) from prediabetic male rats exhibited a greater impairment compared to that in prediabetic females. However, when females became diabetic, they exhibited a greater vascular dysfunction than males. Thus, the aim of this study was to investigate whether the female sex hormone, estrogen preserves mesenteric arterial vasorelaxation in UCD-T2DM female rats at an early prediabetic state. Age-matched female Sprague Dawley and prediabetic (PD) UCD-T2DM rats were ovariectomized (OVX) and subcutaneously implanted with either placebo or 17β-estradiol (E_2, 1.5 mg) pellets for 45 days. We assessed the contribution of endothelium-derived relaxing factors (EDRF) to acetylcholine (ACh)-induced vasorelaxation, using pharmacological inhibitors. Responses to sodium nitroprusside (SNP) and phenylephrine (PE) were also measured. Additionally, metabolic parameters and expression of some targets associated with vascular and insulin signaling were determined. We demonstrated that the responses to ACh and SNP were severely impaired in the prediabetic state (PD OVX) rats, while E₂ treatment restored vasorelaxation in the PD OVX + E₂. Moreover, the responses to PE was significantly enhanced in MA of PD OVX groups, regardless of placebo or E₂ treatment. Overall, our data suggest that 1) the impairment of ACh responses in PD OVX rats may, in part, result from the elevated contractile responses to PE, loss of contribution of endothelium-dependent hyperpolarization (EDH) to vasorelaxation, and a decreased sensitivity of MA to nitric oxide (NO), and 2) the basis for the protective effects of E₂ may be partly attributed to the elevation of the NO contribution to vasorelaxation and its interaction with MA as well as potential improvement of insulin signaling. Here, we provide the first evidence of the role of E₂ in protecting MA from early vascular dysfunction in prediabetic female rats.

Painful diabetic neuropathy leads to functional CaV3.2 expression and spontaneous activity in skin nociceptors of mice

Painful diabetic neuropathy occurs in approximately 20% of diabetic patients with underlying pathomechanisms not fully understood. We evaluated the contribution of the CaV3.2 isoform of T-type calcium channel to hyperglycemia-induced changes in cutaneous sensory C-fiber functions and neuropeptide release employing the streptozotocin (STZ) diabetes model in congenic mouse strains including global knockouts (KOs). Hyperglycemia established for 3-5 weeks in male C57BL/6J mice led to major reorganizations in peripheral C-fiber functions. Unbiased electrophysiological screening of mechanosensitive single-fibers in isolated hairy hindpaw skin revealed a relative loss of (polymodal) heat sensing in favor of cold sensing. In healthy CaV3.2 KO mice both heat and cold sensitivity among the C-fibers seemed underrepresented in favor of exclusive mechanosensitivity, low-threshold in particular, which deficit became significant in the diabetic KOs. Diabetes also led to a marked increase in the incidence of spontaneous discharge activity among the C-fibers of wildtype mice, which was reduced by the specific CaV3.2 blocker TTA-P2 and largely absent in the KOs. Evaluation restricted to the peptidergic class of nerve fibers - measuring KCl-stimulated CGRP release - revealed a marked reduction in the sciatic nerve by TTA-P2 in healthy but not diabetic wildtypes, the latter showing CGRP release that was as much reduced as in healthy and, to the same extent, in diabetic CaV3.2 KOs. These data suggest that diabetes abrogates all CaV3.2 functionality in the peripheral nerve axons. In striking contrast, diabetes markedly increased the KCl-stimulated CGRP release from isolated hairy skin of wildtypes but not KO mice, and TTA-P2 reversed this increase, strongly suggesting a de novo expression of CaV3.2 in peptidergic cutaneous nerve endings which may contribute to the enhanced spontaneous activity. De-glycosylation by neuraminidase showed clear desensitizing effects, both in regard to spontaneous activity and stimulated CGRP release, but included actions independent of CaV3.2. However, as diabetes-enhanced glycosylation is decisive for intra-axonal trafficking, it may account for the substantial reorganizations of the CaV3.2 distribution. The results may strengthen the validation of CaV3.2 channel as a therapeutic target of treating painful diabetic neuropathy.

Relationship between impaired parasympathetic vasodilation and hyposalivation in parotid glands associated with type 2 diabetes mellitus

We examined the relationship between hemodynamics in the three major salivary glands and salivary secretion in urethane-anesthetized and sympathectomized type 2 diabetic and nondiabetic rats via laser speckle imaging and by collecting the saliva. Lingual nerve stimulation elicited rapid increases in glandular blood flow and induced salivary secretion from the three glands in both diabetic and nondiabetic rats. In the parotid gland, the magnitude of blood flow increase and salivary secretion was significantly lower in the diabetic rats when compared with the nondiabetic rats; however, this was not observed in the other glands. Although the intravenous administration of acetylcholine increased blood flow in the parotid gland in a dose-dependent manner, the response was significantly lower in the diabetic rats when compared with the nondiabetic rats. Similarly, mRNA expression levels of M1 and M3 muscarinic acetylcholine receptors in the parotid gland were relatively lower in the diabetic rats compared with the nondiabetic rats. Our results indicate that type 2 diabetes impairs parasympathetic vasodilation and salivary secretion in the parotid gland and suggest that disturbances in the cholinergic vasodilator pathway may contribute to the underlying mechanisms involved in the disruption of parasympathetic nerve-mediated glandular vasodilation.

Salivary gland hypofunction in KK-Ay type 2 diabetic mice

BACKGROUND

Hypofunction of different organs in the body is associated with diabetes, including in the oral cavity. Diabetes is often associated with xerostomia, but the underlying mechanism is not well characterized. Thus, the mechanisms underlying diabetes-induced xerostomia were investigated in this study in KK-A ^y mice as an experimental model of type 2 diabetes.

METHODS

The mechanisms involved in diabetes-induced xerostomia were investigated using the ex vivo glandular perfusion technique, histological analysis, and immunohistochemical and intracellular signaling analyses.

RESULTS

Ex vivo submandibular gland secretions from KK-A^y mice decreased by 30% following stimulation with 0.3 μmol/L carbachol (CCh), a cholinergic agonist. Acinar cell weight was comparable between KK-A^y and control mice, whereas duct cell weight was significantly greater in KK-A^y mice. Concentrations of Na⁺ and Cl^- in the secreted saliva decreased significantly in KK-A^y mice, supporting the finding of increased ductal tissue in KK-A^y mice. Immunohistochemistry revealed no significant differences between KK-A^y and control mice in terms of the expression of Cl^- and water channels, Na⁺ -K⁺ -2Cl^- cotransporters, and membrane proteins critical for fluid secretion. Cellular signaling analysis revealed that the increase in [Ca²⁺ ]_i in response to 0.3 μmol/L CCh was reduced by 30% in KK-A^y mice, although there was no significant difference in the thapsigargin (1.0 μmol/L)-induced increase in store-depleted calcium between KK-A^y and control mice.

CONCLUSIONS

These results demonstrate that submandibular fluid secretion is diminished in KK-A^y mice because of a diminished increase in [Ca²⁺ ]_i . Duct cell weight increased in KK-A^y mice, possibly leading to increased ion reabsorption and thus decreased Na⁺ and Cl^- concentrations in the secreted saliva.