Ca2+: is there something new for the cardiovascular anesthesiologist?

PURPOSE OF REVIEW: Anesthesiologists are frequently called upon to treat abnormalities of heart rhythm or pumping ability. Intracellular Ca is crucial for normal excitation-contraction coupling in the heart and plays a major role in the sequence of events that starts with an electrical signal generated in the atria and ends with myocardial contraction. RECENT FINDINGS: From controlled diffusion within the cell to a potential role as a biological clock, intracellular Ca is receiving a great deal of attention. For example, the pacemaking electrical signal is known to originate in the sinoatrial node myocyte, but exactly what role Ca plays is controversial despite the fact that the sinoatrial node was discovered over 100 years ago. Basic mechanisms involved in disease processes such as atrial fibrillation and new interventions for heart rate control are beginning to emerge. New discoveries in ventricular myocytes are also stimulating the development of promising therapeutic interventions to safely increase the pumping ability of the heart. SUMMARY: As our understanding of cardiac physiology and pharmacology progresses at the subcellular and molecular levels, new therapies will continue to emerge and the practice of anesthesia will benefit greatly.

Halothane, isoflurane, and sevoflurane increase the kinetics of Ca2+-induced conformational change of recombinant human cardiac troponin C.

BACKGROUND: Halothane, isoflurane, and sevoflurane exert negative inotropic side effects, generally mediated via a reduced availability of intracellular calcium. Other possible mechanisms include modified intracellular calcium handling, impaired actomyosin cross-bridge cycling, and/or alteration of calcium-induced conformational changes of the regulatory troponin complex. METHODS: We investigated the effect of halothane, isoflurane, and sevoflurane on calcium-dependent kinetics of isolated human recombinant cardiac troponin C labeled with IAANS (HrcTnC(IAANS)) using stopped-flow and calcium titration techniques. RESULTS: Calcium concentration at half-maximal fluorescence intensity (K(d)) in the control group was 2.1 +/- 0.1 mM. Volatile anesthetics increased calcium sensitivity in a concentration-dependent fashion sevoflurane (K(d) 1.5-1.7 mM, P = 0.001) > halothane (K(d) 1.7-1.9 mM, P < 0.01) > isoflurane (K(d) 1.8-1.9 mM, P < 0.05). The rate constant of conformational changes after rapid dissociation of calcium from HrcTnC(IAANS) (k(off(c))) was moderately prolonged at 4 degrees C by halothane and isoflurane > sevoflurane. CONCLUSION: These mechanisms may counteract the effects of lower calcium availability, and can be responsible for abbreviated, and possibly incomplete, relaxation of cardiac muscle fibers in the presence of volatile anesthetics.

Effects of isoflurane and sevoflurane on intracellular calcium and contractility in pressure-overload hypertrophy.

BACKGROUND: Depression of myocardial contractility as a result of isoflurane appears to be greater in myocardial hypertrophy, and the cellular basis for this difference in susceptibility is not clear. In this study we examined the effects of isoflurane and sevoflurane on contractility and intracellular calcium in an animal model of pressure-overload hypertrophy. METHODS: Pressure-overload hypertrophy was established in young male ferrets by banding the main pulmonary artery for 1 month and the effects of isoflurane and sevoflurane on contractility and intracellular calcium ([Ca]i) were examined in isolated right ventricular papillary muscles, trabeculae, and myocytes. Intracellular calcium was measured with the bioluminescent photoprotein aequorin in isolated papillary muscles, and also with the fluorescent indicator fluo-3 in isolated ventricular myocytes. In addition, Ca sensitivity was assessed in isolated trabeculae after disruption of the surface membrane with a nonionic detergent (skinned fibers). RESULTS: In the presence of isoflurane and sevoflurane, papillary muscles from banded animals exhibited a greater depression of contractility and isolated ventricular myocytes showed a greater decrease in peak [Ca]i. Furthermore, baseline calcium sensitivity was decreased and the slope of the relationship between [Ca] and force was increased in skinned trabeculae from banded animals. Isoflurane decreased calcium sensitivity in trabeculae from both normal and banded animals. CONCLUSIONS: These results suggest that changes in [Ca]i and altered calcium sensitivity are both responsible for the exaggerated effects of some volatile anesthetics on contractility in pressure-overload hypertrophy.

Equilibrium titration of protein-ligand binding affinity in the presence of volatile reagents--a semiautomated approach.

An existing method of equilibrium titration was significantly improved for investigating the effects of volatile anesthetics on Ca(2+) binding characteristics of human recombinant cardiac troponin C in in vitro conditions. The modified method increases stability of volatile compound concentrations in solution and allows for faster and more accurate data acquisition. The time to complete a titration series could be reduced from 28.3 +/- 6.2 min to 9.3 +/- 2.1 min, whereas the dispersion for pK(d) was decreased from 2.16 +/- 0.27 to 0.63 +/- 0.27. The method utilizes a semiautomatic approach to continuously monitor stability of fluorescence signals in a sealed chamber with greatly reduced air space.

Effects of volatile anesthetics on elastic stiffness in isometrically contracting ferret ventricular myocardium.

The effects of halothane, isoflurane, and sevoflurane on elastic stiffness, which reflects the degree of cross-bridge attachment, were studied in intact cardiac muscle. Electrically stimulated (0.25 Hz, 25 degrees C), isometrically twitching right ventricular ferret papillary muscles (n = 15) at optimal length (L(max)) were subjected to sinusoidal length oscillations (40 Hz, 0.25- 0.50% of L(max) peak to peak). The amplitude and phase relationship with the resulting force oscillations was decomposed into elastic and viscous components of total stiffness in real time. Increasing extracellular Ca(2+) concentration in the presence of anesthetics to produce peak force equal to control increased elastic stiffness during relaxation, which suggests a direct effect of halothane and sevoflurane on cross bridges.

Functional analysis of a troponin I (R145G) mutation associated with familial hypertrophic cardiomyopathy.

Familial hypertrophic cardiomyopathy has been associated with several mutations in the gene encoding human cardiac troponin I (HCTnI). A missense mutation in the inhibitory region of TnI replaces an arginine residue at position 145 with a glycine and cosegregates with the disease. Results from several assays indicate that the inhibitory function of HCTnI(R145G) is significantly reduced. When HCTnI(R145G) was incorporated into whole troponin, Tn(R145G) (HCTnT small middle dotHCTnI(R145G) small middle dotHCTnC), only partial inhibition of the actin-tropomyosin-myosin ATPase activity was observed in the absence of Ca(2+) compared with wild type Tn (HCTnT small middle dotHCTnI small middle dotHCTnC). Maximal activation of actin-tropomyosin-myosin ATPase in the presence of Ca(2+) was also decreased in Tn(R145G) when compared with Tn. Using skinned cardiac muscle fibers, we determined that in comparison with the wild type complex 1) the complex containing HCTnI(R145G) only inhibited 84% of Ca(2+)-unregulated force, 2) the recovery of Ca(2+)-activated force was decreased, and 3) there was a significant increase in the Ca(2+) sensitivity of force development. Computer modeling of troponin C and I variables predicts that the primary defect in TnI caused by these mutations would lead to diastolic dysfunction. These results suggest that severe diastolic dysfunction and somewhat decreased contractility would be prominent clinical features and that hypertrophy could arise as a compensatory mechanism.