CALL FOR PAPERS Sex and Gender Differences in Cardiovascular Physiology—Back to the Basics Cardiac contraction, calcium transients, and myofilament calcium sensitivity fluctuate with the estrous cycle in young adult female mice

MacDonald JK, Pyle WG, Reitz CJ, Howlett SE. Cardiac contraction, calcium transients, and myofilament calcium sensitivity fluctuate with the estrous cycle in young adult female mice. Am J Physiol Heart Circ Physiol 306: H938–H953, 2014. First published January 24, 2014; doi:10.1152/ajpheart.00730.2013.—This study established conditions to induce regular estrous cycles in female C57BL/6J mice and investigated the impact of the estrous cycle on contractions, Ca transients, and underlying cardiac excitation-contraction (EC)-coupling mechanisms. Daily vaginal smears from group-housed virgin female mice were stained to distinguish estrous stage (proestrus, estrus, metestrus, diestrus). Ventricular myocytes were isolated from anesthetized mice. Contractions and Ca transients were measured simultaneously (4 Hz, 37°C). Interestingly, mice did not exhibit regular cycles unless they were exposed to male pheromones in bedding added to their cages. Field-stimulated myocytes from mice in estrus had larger contractions ( 2-fold increase), larger Ca transients ( 1.11-fold increase), and longer action potentials ( 2-fold increase) compared with other stages. Larger contractions and Ca transients were not observed in estrus myocytes voltage-clamped with shorter action potentials. Voltage-clamp experiments also demonstrated that estrous stage had no effect on Ca current, EC-coupling gain, diastolic Ca , sarcoplasmic reticulum (SR) Ca content, or fractional release. Although contractions were largest in estrus, myofilament Ca sensitivity was lowest (EC50 values 1.15-fold higher) in conjunction with increased phosphorylation of myosin binding protein C in estrus. Contractions were enhanced in ventricular myocytes from mice in estrus because action potential prolongation increased SR Ca release. These findings demonstrate that cyclical changes in reproductive hormones associated with the estrous cycle can influence myocardial electrical and contractile function and modify Ca homeostasis. However, such changes are unlikely to occur in female mice housed in groups under conventional conditions, since these mice do not exhibit regular estrous cycles.