Stretch as a Regulator of Cardiac Contractility: Mechanisms Associated with the Increased Intracellular Calcium Transients

Abstract

It is well known that stretch is an important regulator of cardiac function, and that an increase in LV diastolic volume increases the subsequent force of contraction. The response to stretch is biphasic, with an initial increase in force that coincides with the stretch attributed to changes in myofilament overlap as well as Ca2+ sensitivity. A second phase slowly develops over a period of minutes, known as the slow force response, or SFR, which is accompanied by a concomitant increase in the amplitude of the Ca2+ transients. The aim of this thesis was to investigate the mechanisms underlying the SFR. To achieve this, right ventricular trabeculae from adult Wistar rats were used, and measurements of contractile force, intracellular Ca2+ and intracellular pHi were made at a physiological temperature of 37ºC. Experiments show that the magnitude of the SFR was dependent on the rate of stimulation and [Ca2+]o, such that the stretch response was the greatest at a reduced SR Ca2+ content. Furthermore, this study showed, for the first time, that the addition of cross-bridge cycle inhibitors (20 mM BDM or 20 μM blebbistatin) were able to significantly reduce the increased Ca2+ transient associated with the SFR. The SFR was blocked by SN-6, a reverse mode Na+/Ca2+-exchange (NCX) inhibitor. Additionally, the SFR was reduced in the presence of Na+/H+-exchange (NHE) inhibitor HOE642 in HCO3 −-free solutions, but remained unaffected in HCO3 −-buffer solutions. The SFR was increased when HCO3 − transport was prevented using DIDS. Together, these results suggest a mechanism in which increased metabolic demand from stretch gives rise to increased intracellular Na+ accumulation via NHE and the Na+/ HCO3 − transporters. This thesis also explored the possibility of stretch-dependent autocrine/paracrine factors which could contribute to the increased Ca2+ transients during the SFR. To do this, a bioassay system was utilised in which the coronary effluent from an ‘upstream’ stretched, isolated whole heart was collected, and was used to superfuse a separate ‘downstream’ isolated cardiac trabecula. The ‘stretched’ coronary effluent exerted a positive inotropic response on the trabecula. Using liquid chromatography-mass spectroscopy (LC/MS), the inotropic factor of interest was identified to be prostaglandin F2α.