ryanodine and Hyperglycemia

1. Previously, we found that contractions in response to receptor-dependent (i.e. a(1)-adrenoceptor agonist phenylephrine) and -independent (i.e. cyclopiazonic acid) stimuli are decreased in rat aorta during late diabetes. The aim of the present study was to further investigate the changes of intracellular Ca(2+) homeostasis in diabetic aortic smooth muscle cells. Functional changes of inositol 1,4,5-trisphosphate (IP(3))- and ryanodine-sensitive Ca(2+) stores of the sarcoplasmic reticulum (SR) were evaluated using Fluo-3 acetoxymethyl ester fluorescence, western blot and organ bath techniques. 2. In aortic smooth muscle cells from diabetic rats, the Ca(2+) release and Ca(2+) influx caused by both 10 mmol/L phenylephrine (depletion of IP(3)-sensitive Ca(2+) stores) and 1 mmol/L ryanodine (depletion of ryanodine-sensitive Ca(2+) stores) were both significantly decreased compared with control. Moreover, protein expression levels of IP(3) (260 kDa) and ryanodine receptors (500 kDa) were reduced by 31.8 +/- 7.7 and 69.2 +/- 8.4%, respectively, in aortas from diabetic rats compared with those from control rats. 3. In diabetic rat aorta, phenylephrine-induced contractility was decreased to approximately two-thirds of that in controls, whereas ryanodine alone did not cause obvious contraction in aortas from either control or diabetic rats. 4. The present results suggest that the hyporeactivity of aortic smooth muscle to vasoconstrictors in diabetes results mainly from changes to the IP(3)-sensitive Ca(2+) release pathway. The SR Ca(2+) signalling pathway plays a crucial role in the development of diabetic vascular complications.

Topics: Animals; Aorta; Calcium; Calcium Signaling; Diabetes Mellitus, Experimental; Hyperglycemia; Indoles; Inositol 1,4,5-Trisphosphate; Male; Muscle Contraction; Myocytes, Smooth Muscle; Phenylephrine; Rats; Rats, Wistar; Ryanodine; Sarcoplasmic Reticulum

Pancreatic beta-cells from obese-hyperglycemic (ob/ob) mice are widely used for studying the mechanisms of insulin release, including its regulation by the cytoplasmic Ca2+ concentration ([Ca2+]i). In this study, we compared changes of [Ca2+]i in single beta-cells isolated from ob/ob mice with those from lean mice using dual-wavelength microfluorometry and the indicator fura-2. There were no differences in the frequency, amplitude, and half-width of the slow oscillations induced by glucose. Most beta-cells from the obese mice responded to 10 mM caffeine with transformation of the oscillations into sustained elevation of [Ca2+]i, a process counteracted by ryanodine. The beta-cells from the obese mice were characterized by ample generation of [Ca2+]i transients, which increased in number in the presence of glucagon. The transients became less frequent when leptin was added at a concentration as low as 1 nM. It is suggested that the excessive firing of [Ca2+]i transients in the ob/ob mice is owing to the absence of leptin and is mediated by activation of the phospholipase C signaling pathway.

Topics: Animals; Caffeine; Calcium; Cytoplasm; Glucagon; Hyperglycemia; Islets of Langerhans; Leptin; Mice; Mice, Inbred C57BL; Mice, Obese; Obesity; Ryanodine; Signal Transduction; Type C Phospholipases