CALCIUM AND MECHANICALLY INDUCED POTENTIALS IN FIBROBLASTS OF RAT ATRIUM

Objectives: Electrically non-excitable cardiac fibroblasts in the sino -atrial node region are mechano-sensitive. Rhythmic contraction of adj acent myocardium, or artificial stretch of the tissue, produce a rever sible change in the membrane potential: mechanically induced potential s (MIP). Stretch of normal cardiomyocytes can be associated with intra cellular calcium changes, The purpose of this study is to use pharmaco logical interventions to investigate the possibility that stretch-indu ced Ca2+ entry through ion channels in the sarcolemma and Ca2+ release from internal stores play a role in MIP generation. Methods: Isolated spontaneously contracting or artificially stretched preparations of r ight atrium of rat heart were superfused with physiological solutions. An intracellular floating microelectrode recorded fibroblast MIPs and was also used for injection of current. A dye, Lucifer yellow, applie d through the micropipette, identified recording sites. We assessed th e role of extracellular Ca2+ using EGTA in the bathing solution. For t he role of intracellular Ca2+ in the generation of MIP, several substa nces that influence [Ca2+](i) handling were applied intracellularly by diffusion from the recording microelectrode. These include: BAPTA (to chelate intracellular Ca2+); BHQ, thapsigargin and CPA (to deplete Ca 2+ from intracellular stores by inhibition of the endoplasmic reticulu m (ER) ATP Ca2+ pump), and caffeine and ryanodine (to induce ER Ca2+ r elease). Results: All the pharmacological compounds which were introdu ced intracellulary, and EGTA applied extracellularly, decreased the am plitude of the MIP to variable degrees. Only thapsigargin induced a bi -phasic response with an initial increase in MIP amplitude, followed b y a decrease. MIP duration was reduced by most interventions, exceptio ns being low extracellular Ca2+, BHQ and ryanodine, Short duration ext racellular application of caffeine, which was added to the perfusate a s a secondary contractile stimulus, partly restored the MIPs by activa tion of cardiac contraction. Intracellular current injection, before a ny intervention, linearly altered both membrane potential (E(m)) and M IP amplitude (V-m). Application of compounds listed above introduced n on-linearity to the E(m)/V-m relationship. Conclusion: We suggest that mechanically induced Ca2+ influx, induced through stretch-activated c hannels in the plasma membrane, and release of Ca2+ from the endoplasm ic reticulum, play key roles in the mechanism of MIP generation. Furth er, our results demonstrate the existence of functional ryanodine/caff eine-sensitive Ca2+ stores in cardiac fibroblasts.