Programme Contents

Cardiomyopathies: When is Ca²⁺ the culprit?

The coordinated contraction and relaxation of the heart that allows it to function as a pump arises from transient changes in the concentration of Ca²⁺ within the myocytes. Influx of Ca²⁺ during the cardiac action potential triggers release from the intracellular Ca²⁺ store (the sarcoplasmic reticulum, or SR), rapidly increasing the cytosolic [Ca²⁺] ∼10-fold. Ca²⁺ then activates cross-bridge cycling, and force production, by binding to the regulatory sites on troponin C. Relaxation takes place when the [Ca²⁺] returns again to resting levels as it is removed from the cytosol by two principle transport mechanisms: re-uptake into the SR by the Ca²⁺-ATPase (SERCA2a); and transport across the sarcolemma (SL) by the Na⁺/Ca²⁺ exchanger (NCX). Given the key role of Ca²⁺ in the mechanical activity of the heart, it is not surprising that Ca²⁺ mis-handling is often implicated in cardiomyopathies where force production is compromised. However, many other changes also occur in hearts en route to failure, such as extracellular matrix remodelling and increased ß-adrenergic stimulation. The impact of these changes on a beat-to-beat basis remains unclear, particularly since most studies only examine force and Ca²⁺ during steady-state, or single beat responses. In this study, we utilised an animal model of hypertensive failure to gain insights into Ca²⁺ homeostasis in the recovery from non-steady-state interventions in isolated left ventricular preparations.

Measurements of isometric force and [Ca²⁺]_i were made at 37°C in left ventricular trabeculae from failing spontaneously hypertensive rat (SHR) hearts, and their normotensive Wistar-Kyoto (WKY) controls. At 1Hz, peak stress was reduced in SHR (14.5 ± 2.4 mN mm⁻² versus 22.5 ± 6.7 mN mm^-2 for WKY), although the Ca²⁺ transients were bigger (peak [Ca²⁺]_i 0.60 ± 0.08 μM versus 0.38 ± 0.03 μM for WKY) with a slower decay of fluorescence (time constant 0.105 ± 0.005 s versus 0.093 ± 0.002 s for WKY). Two experimental protocols were used to potentiate force as a probe of dynamic Ca²⁺ cycling: (i) an interval of 30s rest, and (ii) a 30s train of paired-pulses, and the recirculation fraction (RF) calculated for recovery to steady-state. No difference was found between rat strains for RF calculated from either peak force or Ca²⁺, although the RF was dependent on potentiation protocol. Since SR uptake is slower in SHR, the lack of change in RF must be due to a parallel decrease in trans-sarcolemmal Ca²⁺ extrusion. This view was supported by a slower decay of caffeine-induced Ca²⁺ transients in SHR trabeculae. Confocal analysis of LV free wall showed t-tubules were distorted in SHR myocytes, with reduced intensity of SERCA2a and NCX labelling in comparison to WKY.