Axial stretch enhances sarcoplasmic reticulum Ca²⁺ leak and cellular Ca²⁺ reuptake in guinea pig ventricular myocytes: Experiments and models

Cardiac cellular calcium (Ca²⁺) handling is the well-investigated mediator of excitation-contraction coupling, the process that translates cardiac electrical activation into mechanical events. The reverse-effects of mechanical stimulation on cardiomyocyte Ca²⁺ handling-are much less well understood, in particular during the inter-beat period, called 'diastole'. We have investigated the effects of diastolic length changes, applied axially using a pair of carbon fibres attached to opposite ends of Guinea pig isolated ventricular myocytes, on the availability of Ca²⁺ in the main cellular stores (the sarcoplasmic reticulum; SR), by studying the rest-decay of SR Ca²⁺ content [Ca²⁺]_SR, and the reloading of the SR after prior depletion of Ca²⁺ from the cell. Cells were loaded with Fura-2 AM (an indicator of the cytosolic 'free' Ca²⁺ concentration, [Ca²⁺]_i), and pre-conditioned by field-stimulation (2 Hz) at 37 °C, while [Ca²⁺]_i transients and sarcomere length (SL) were recorded simultaneously. After reaching a steady state in the behaviour of observed parameters, stimulation was interrupted for between 5 and 60 s, while cells were either held at resting length, or stretched (controlled to cause a 10% increase in SL, to aid inter-individual comparison). Thereafter, each cell was returned to its original resting length, followed by swift administration of 10 mM of caffeine (in Na⁺/Ca²⁺-free solution), which causes the release of Ca²⁺ from the SR (caffeine), but largely prevents extrusion of Ca²⁺ from the cytosol to the cell exterior (Na⁺/Ca²⁺-free solution). By comparing the [Ca²⁺]_i in cells exposed/not exposed to diastolic stretch of different duration, we assessed the rest-decay dynamics of [Ca²⁺]_SR. To assess SR reloading after initial Ca²⁺ depletion, the same stretch protocol was implemented after prior emptying of the cell by application of 10 mM of caffeine in normal Tyrode solution (which causes Ca²⁺ to be released from the SR and extruded from the cell via the Na⁺/Ca²⁺ exchanger; NCX). Axial stretch enhanced the rate of both rest-decay and reloading of [Ca²⁺]_SR. Application of 40 μM streptomycin, a blocker of stretch-activated ion channels, did not affect the stretch-induced increase in SR reloading. This behaviour was reproduced in a computer simulation study, using a modified version of the 2006 Iribe-Kohl-Noble model of single cardiac myocyte Ca²⁺ handling, suggesting that stretch increases both Ca²⁺ leak from the SR and Ca²⁺ influx via the sarcolemma. This may have important implications for the mobilisation of Ca²⁺ in stretched cells, and could contribute to the regional 'matching' of individual cardiomyocyte contractility to dynamic, and regionally varying, changes in mechanical loads, such as diastolic pre-load, of cardiac tissue.