Myofilament sensitivity to Ca²⁺ in ventricular myocytes from the Goto-Kakizaki diabetic rat

The chronic effects of type 2 diabetes mellitus on myofilament sensitivity to Ca²⁺ in ventricular myocytes from the Goto-Kakizaki (GK) rat have been investigated. Experiments were performed in ventricular myocytes isolated from 17-month GK rats and age-matched Wistar controls. Myocytes were loaded with fura-2 (an indicator for intracellular Ca²⁺ concentration) and the fura-2 ratio (340/380 nm), and shortening were measured simultaneously in electrically stimulated myocytes. Myofilament sensitivity to Ca²⁺ was assessed from phase-plane diagrams of fura-2 versus cell length by measuring the gradient of the fura-2-cell length trajectory during late relaxation of the twitch contraction. Non-fasting and fasting blood glucose were elevated in GK rats compared to controls. Fasting blood glucose was 151.5 ± 15.3 mg/dl (n = 8) in GK rats compared to 72.1 ± 3.6 mg/dl (n = 9) in controls. At 120 min after intraperitoneal injection of glucose (2 g/kg body weight), blood glucose was 570.8 ± 36.8 mg/dl in GK rats compared to 148 ± 8.6 mg/dl in controls. Amplitude of shortening was significantly increased in myocytes from GK rats (6.56 ± 0.54%, n = 31) compared to controls (5.05 ± 0.43%, n = 36), and the amplitude of the Ca²⁺ transient was decreased in myocytes from GK rats (0.23 ± 0.02 RU, n = 31) compared to controls (0.30 ± 0.02 RU, n = 36). The fura-2-cell length trajectory during the late stages of relaxation of the twitch contraction was steeper in myocytes from GK rats (89.2 ± 16.6 μm/RU, n = 27) compared to controls (31.9 ± 5.9 μm/RU, n = 35). Increased amplitude of shortening, accompanied by a decrease in amplitude of the Ca²⁺ transient, might be explained by an increased myofilament sensitivity to Ca²⁺.