MLO-Y4 Fluid Flow Shear Stress Conditioned Media Enhances Cardiac Contractility and Intracellular Ca2.

The skeleton is in complex interplay with the other systems of the body and is highly responsive to input from the external environment. Bone mechanical loading results in interstitial fluid flow via the lacunar canalicular system, generating fluid flow sheer stress (FFSS). FFSS variably stresses osteocytes, subsequently causing the release of metabolites and protein factors which function locally to increase bone formation and may play a role in crosstalk between various organ systems, for instance between bone and skeletal muscle. Therefore, we hypothesized that this crosstalk includes altering cardiac function. To test this hypothesis, media conditioned by MLO-Y4 osteocyte-like cell culture line under FFSS was used to model the endocrine effects of bone during mechanical loading on contraction of ex vivo Langendorf-perfused isolated hearts. When hearts were externally paced at a fixed rate, FFSS osteocyte conditioned media (CM) induced significant premature contractions compared to vehicle (control). FFSS osteocyte CM administration to self-paced hearts increased total contraction force by 31%. To determine if the mechanism involved intracellular Ca²⁺, vehicle and FFSS bone CM were perfused over cultured H9C2 cardiomyocytes while undergoing Ca²⁺ imaging using Fluo-8. We observed an increase in intracellular Ca²⁺ with FFSS CM perfusion of cardiomyocytes compared to vehicle. These increases were only present with exogenous electrical pacing. Our findings demonstrate that FFSS bone CM enhances cardiac contractility by increasing intracellular cardiomyocyte Ca²⁺. The results obtained in this study suggest that the skeleton, responding to mechanical strain, has the potential to augment cardiac output and provide evidence for bone-heart crosstalk.