Myocardial Effects of Halothane and Sevoflurane in Diabetic Rats

Background Diabetes induces significant myocardial abnormalities, but the effects of halogenated anesthetics on this diseased myocardium remain a matter of debate. Methods Left ventricular papillary muscles and triton-skinned cardiac fibers were provided from control and streptozotocin-induced diabetic rats. The effects of halothane and sevoflurane were studied on inotropic and lusitropic responses, under low (isotony) and high (isometry) loads in papillary muscles and then on isometric tension-Ca2+ concentration (pCa) relations obtained in triton-skinned cardiac fibers. Data are presented as mean +/- SD. Results Sevoflurane and halothane induced a negative inotropic effect that was more important in diabetic rats (active force: 1.5% halothane, 19+/-6 vs. 24+/-6% of baseline, P < 0.05; 3.6% sevoflurane, 47+/-14 vs. 69+/-17% of baseline, P < 0.05). However, when differences in minimum alveolar concentration were considered, no significant difference was observed between groups for halothane. The effects of halothane and sevoflurane on isotonic relaxation and postrest potentiation were not significantly different between groups. In contrast, the decrease in Ca myofilament sensitivity produced by each anesthetic agent was greater in diabetic rats than in control rats (0.65% halothane, -0.15+/-0.07 vs. -0.05+/-0.04 pCa unit, P < 0.05; 1.8% sevoflurane, -0.12+/-0.06 vs. -0.06+/-0.04 pCa unit, P < 0.05). Conclusions The negative inotropic effect of halothane and sevoflurane was greater in diabetic rats, mainly because of a significant decrease in myofilament Ca sensitivity.

Ca2+ measurements in skinned cardiac fibers: effects of Mg2+ on Ca2+ activation of force and fiber ATPase

In contrast to previous studies, a new fluorescent method was used to accurately determine the Ca2+ concentration in test solutions used to activate skinned rat cardiac cells. This method used the calcium green-2 fluorescent indicator, which is shown to change its fluorescence over the Ca2+ range responsible for Ca2+ activation of force and ATPase. The dissociation constant ( K d) of calcium green-2 for Ca2+ was determined for three different Mg2+ concentrations in solutions similar to those used in the experiment. Increasing Mg2+ concentration from 1.0 to 8.0 mM had no significant effect on the Ca2+sensitivity of either force or actomyosin ATPase activity, in contrast to previous reported studies on force. The ATPase activity was activated at lower Ca2+ concentration than the force. The ratio (ATPase/force) is proportional to the dissociation rate of force-generating myosin cross bridges and decreased during Ca2+ activation. These findings are consistent with the hypothesis that cardiac muscle contraction is activated by a single Ca2+-specific binding site on troponin C.

The effect of collagenase and temperature on mitochondrial respiratory parameters in saponin-skinned cardiac fibers

The results of a comparative study of the respiration rates of mitochondria in saponin-skinned rat cardiac fibers (SF) and in fibers treated with saponin and collagenase (SCF) suggest that only about half of the whole population of mitochondria manifest their activity in SF, in contrast to SCF, in response to extracellular substrates of oxidative phosphorylation. The apparent Km value for ADP with succinate as substrate, which was as high as 330±32 μM in SF in SF at 20 °C, decreased about 2-fold in SCF at the same temperature and in SF at 37 °C, and decreased further to 67±8 μM in SCF at 37 °C. Thus, weakening or breaking of cellular contacts by collagenase and the temperature-dependence of diffusion of substrates such as ADP, seem to be important factors that determine the respiratory activity and regulatory parameters of mitochondria in saponin-permeabilized cardiomyocytes.

Hypertrophy alters effect of Ins(1,4,5)P3 on Ca2+ release in skinned rat heart muscle

The effects of D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] on the ability of the sarcoplasmic reticulum (SR) to accumulate and release Ca2+ and on the Ca2+ sensitivity of the contractile proteins were investigated using chemically (saponin) skinned cardiac fibers (60–120 microns diam) obtained from normal and pressure-overloaded hypertrophied rat left ventricles. Left ventricular pressure overload was induced by partial ligation of the abdominal aorta 3-6 wk before study. Age- and weight-matched normal rats served as controls. Pressure overload increased the left ventricular weight-to-body weight ratio by 45%. Ins(1,4,5)P3 at a concentration of 10 microM did not change the Ca(2+)-tension relationship at Ca2+ concentrations of 10(-7) to 10(-5) M in either normal or hypertrophied fibers. Ins(1,4,5)P3 also did not influence Ca2+ uptake by the SR in either normal or hypertrophied fibers. Ins(1,4,5)P3 did not induce Ca2+ release from the SR directly in either group. However, pretreatment with Ins(1,4,5)P3 enhanced the 5 mM caffeine-induced Ca2+ release by 80.5 +/- 22.7% in normal fibers enhances, rather than directly induces, SR Ca2+ release in normal rat hearts and that sustained pressure overload diminishes the response of the SR Ca(2+)-release system to Ins(1,4,5)P3, an action that may be partly responsible for contractile dysfunction in cardiac hypertrophy.

Effect of acidosis on contractile system in skinned fibers of hypertrophied rat heart

Hypertrophied hearts have enhanced susceptibility to ischemia-induced contractile dysfunction. To explore the mechanisms of this phenomenon, we studied the effect of acidosis on the Ca2+ sensitivity of the contractile proteins and on Ca2+ accumulation by the sarcoplasmic reticulum (SR) in chemically (saponin) skinned cardiac fibers obtained from normal and pressure-overloaded hypertrophied rat left ventricles. Left ventricular pressure overload was induced by partial ligation of the abdominal aorta 6-8 wk before study. Age- and weight-matched normal rats served as controls. Pressure overload increased the left ventricular weight-to-body weight ratio by 48%. Reduction in pH shifted the pCa-tension curve to the right similarly in normal and hypertrophied preparations, and there was no difference in pCa-tension relationship at pH 7.0 or 6.5 between the two groups. However, reducing the pH of 1 microM Ca2(+)-loading solution from 7.0 to 6.5 decreased the amount of Ca2+ accumulated in the SR to 66.2 +/- 3.0% in normal fibers and 43.2 +/- 4.0% in hypertrophied fibers (P less than 0.01). We conclude that the enhanced susceptibility of hypertrophied hearts to ischemia-induced diastolic dysfunction may be partly explained by the greater depressing effect of acidosis on Ca2+ accumulation by the SR.

Sarcoplasmic reticulum function in skinned fibers of hypertrophied rat ventricle

This study was designed to examine the Ca2+ sensitivity of the contractile system and the ability of the sarcoplasmic reticulum (SR) to accumulate and release Ca2+ in chemically (saponin) skinned cardiac fibers obtained from normal and pressure-overloaded hypertrophied rat left ventricles. Left ventricular pressure overload was induced by partial ligation of the abdominal aorta 6-8 wk before study. Age- and weight-matched normal rats served as controls. Pressure over-load increased the left ventricular weight-to-body weight ratio by 51%. There were no differences in the Ca2+-tension relationship between normal and hypertrophied preparations at Ca2+ concentrations of 10(-7) to 10(-4) M. Caffeine-induced Ca2+ release from the maximally Ca2+ -loaded SR was also not different between the two groups at caffeine concentrations of 0.5-30 mM. However, when the relative amount of Ca2+ accumulated in the SR with 10(-6), 3 x 10(-6), or 10(-5) M Ca2+ loading solutions for various loading periods was estimated by the area under the 25 mM caffeine-induced contraction, the accumulation of Ca2+ was significantly slower in hypertrophied fibers than in normal fibers. We conclude that depressed Ca2+ accumulation by the SR plays a role in modulation of contractile performance in this model of chronic pressure overload in rats.