Myocardial force and intracellular Ca2+ in an animal model of hypertensive heart failure

Abstract

Hypertensive heart failure has long been associated with diminished cardiac contractile function, yet the underlying cellular mechanisms are not well understood. The aim of this Thesis was to investigate the relationship between intracellular calcium ([Ca2+]i) and isometric force during the relatively narrow time frame in which long-standing compensated hypertrophy progresses to decompensated end-stage heart failure in an animal model of human essential hypertension. In order to carry out this aim, left ventricular trabeculae were utilized from failing hearts of spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto (WKY) controls. At a physiological stimulation frequency (5 Hz), and temperature (37 °C), the peak stress of SHR trabeculae was significantly reduced compared to WKY, although no differences in the time-course of the twitch were detected. Measurements using fura-2/AM as an index of intracellular [Ca2+] showed that, for SHR, both the peak of the Ca2+ transient and the resting [Ca2+]i were increased and the decay of the Ca2+ transient was prolonged compared to WKY. This unexpected result, i.e. depression of twitch force despite an increased Ca2+ transient, was investigated further by utilizing experimental protocols known to affect [Ca2+]i and force. Varying extracellular calcium ([Ca2+]o) between 0.5 and 5 mM showed that the reduction of force development by SHR trabeculae was not associated with reduced myofilament Ca2+ sensitivity, since, although peak [Ca2+]i continued to increase with increasing [Ca2+]o, peak stress reached a plateau. Investigation of the force-frequency response between 0.2 and 10 Hz showed that the mismatch in peak Ca2+ and peak force was apparent across all frequencies for SHR. A consistent finding of studies that have made measurement of [Ca2+]i in failing myocardium is that the decay of intracellular Ca2+ following SR release is prolonged. Additionally, expression levels of the SR Ca2+ -ATPase have been reported as reduced, in conjunction with increased expression of the sarcolemmal Na+/Ca2+ exchanger. Although the decay of fluorescence was slower for SHR in this study, no experimental evidence was found to suggest that sarcolemmal Ca2+ extrusion was increased in SHR in comparison to WKY. The re-circulation fraction of activator Ca2+ during recovery from potentiation was not different between rat strains, indicating that SL Ca2+ extrusion was not increased in SHR. Additionally, the decay of fluorescence remained slower for SHR even when the SR Ca2+ -ATPase contribution was functionally removed. Inhibition of the SL Ca2+ -ATPase, together with the functional removal of the SR, removed the differences in the decay of fluorescence between rat strains. A decrease in the sarcolemmal extrusion of [Ca2+]i by the Ca2+ -.ATPase might therefore explain the observed differences in the resting [Ca2+]i and in the amplitude of the Ca2+ transient between rat strains. In summary, this study has provided the first measurements of [Ca2+]i and isometric force carried out at physiological temperature and stimulation frequency in LV trabeculae from failing SHR hearts and their age-matched, normotensive, WKY controls. Most importantly, for this animal model the contractile dysfunction typical of heart failure is not associated with reduced availabilty of [Ca2+]i. Instead it is suggested that contractile function is compromised in these LV trabeculae by the increased collagen, and its three-dimensional organisation.