Altered inotropic response to IGF-I in diabetic rat heart: influence of intracellular Ca²⁺and NO

Abstract

Normally, insulin-like growth factor I (IGF-I) exerts positive effects on cardiac growth and myocardial contractility, but resistance to its action has been reported in diabetes. This study was designed to determine whether IGF-I-induced myocardial contractile action is altered in diabetes as a result of an intrinsic alteration of contractile properties at the cellular level. Contractile responses to IGF-I were examined in left ventricular papillary muscles and ventricular myocytes from normal and short-term (5–7 days) streptozotocin-induced diabetic rats. Mechanical properties of muscles and myocytes were evaluated using a force transducer and an edge detector, respectively. Preparations were electrically stimulated at 0.5 Hz, and contractile properties analyzed include peak tension development (PTD) or peak twitch amplitude (PTA), time to peak contraction/shortening, and time to 90% relaxation/relengthening. Intracellular Ca2+ transients were measured as fura 2 fluorescence intensity changes. IGF-I (1–500 ng/ml) caused a dose-dependent increase in PTD and PTA in preparations from normal but not diabetic animals. IGF-I did not alter time to peak contraction/shortening or time to 90% relaxation/relengthening. Pretreatment with the NO synthase inhibitor N ω-nitro-l-arginine methyl ester (100 μM) attenuated IGF-I-induced increases in PTD in normal myocardium but unmasked a positive inotropic action in diabetic animals. Pretreatment with N ω-nitro-l-arginine methyl ester blocked IGF-I-induced increases in PTA in single myocytes. Consistent with its inotropic actions on muscles and myocytes, IGF-I induced a dose-dependent increase in Ca2+transients in normal but not diabetic myocytes. These results suggest that the IGF-I-induced inotropic response is depressed in diabetes because of an intrinsic alteration at the myocyte level. Mechanisms underlying this alteration in IGF-I-induced myocardial response may be related to changes in intracellular Ca2+ and/or NO production in diabetes.

Leptin Attenuates Cardiac Contraction in Rat Ventricular Myocytes

“…Single ventric-ular myocytes were isolated from adult male Sprague-Dawley rats (200 to 225 g) as described previously. 18 Briefly, the animals were euthanized, and their hearts were rapidly removed and perfused (at 37°C) with oxygenated (5%CO 2 25, HEPES 10, and glucose 11.1, pH 7.4). Hearts were subsequently perfused with a nominally Ca 2ϩ -free KHB buffer for 2 to 3 minutes until spontaneous contractions ceased, followed by a 20-minute perfusion with Ca 2ϩ -free KHB containing 223 U/mL collagenase (Worthington Biochemical Corp) and 0.1 mg/mL hyaluronidase (Sigma Chemical Co).…”

“…18 In brief, coverslips with cells attached were placed in a chamber mounted on the stage of an inverted microscope (Olympus X-70) and superfused (Ϸ2 mL/min at 25°C) with a buffer containing (mmol/L) NaCl 131, KCl 4, CaCl 2 1, MgCl 2 1, glucose 10, and HEPES 10, at pH 7.4. The cells were field-stimulated at a frequency of 0.5 Hz, 3 ms in duration.…”

“…Fluorescence decay time () was also measured as an indication of the intracellular Ca 2ϩ clearing rate. 18 …”

Influence of Gender on Intrinsic Contractile Properties of Isolated Ventricular Myocytes from Calmodulin‐Induced Diabetic Transgenic Mice

“…One of the most prominent complications of diabetes is impaired ventricular contraction and relaxation, which serve as the hallmarks of diabetic cardiomyopathy (3). Our previous studies using chemically-induced and genetically-predisposed diabetic rodent models have revealed prolonged duration and reduced rate of both contraction and relaxation in papillary muscle as well as ventricular myocytes (4)(5)(6)(7). However, the exact nature underscoring the pathogenesis of diabetic cardiomyopathy has not been elucidated.…”

IGF-I attenuates diabetes-induced cardiac contractile dysfunction in ventricular myocytes

“…Diastolic dysfunction is the most prominent defect of diabetic cardiomyopathy, manifested by decreased compliance, prolonged myocardial relaxation, and altered intracellular Ca 2ϩ homeostasis (17,32,33). Recent studies suggest that diabetic cardiomyopathy may be directly due to impaired mechanical function at the single ventricular myocyte level in both chemically induced and genetically predisposed experimental diabetic models (32,33,36).…”