Mechanisms associated with the negative inotropic effect of deuterium oxide in single rat ventricular myocytes

Deuterium oxide (D2O) is known to cause a. negative inotropic effect in mus cle although the mechanisms associated with this response in cardiac muscle are not well understood. We studied the effects of D2O in single rat ventr icular myocytes in order to characterise the mechanisms associated with its negative inotropic effect and to assess its possible use as an acute modul ator of microtubules. D2O rapidly reduced the magnitude of contraction in r at ventricular myocytes, and there was some recovery of contraction in the presence of D2O. Colchicine, an agent known to depolymerise microtubules, d id not modify the effect of D2O. D2O decreased the L-type Ca2+ current (I-C a), measured under whole cell and perforated patch clamp conditions. Slowin g of the time to peak and a delay in inactivation of I-Ca were observed. In tracellular calcium ([Ca2+](i)) and sodium ([Na+](i)) were measured using t he fluorescent indicators fura-2 and SBFI, respectively. The fall in contra ction upon exposure to D2O was not associated with a fall in the [Ca2+](i) transient; this response is indicative of a reduction in myofilament Ca2+ s ensitivity. Both the [Ca2+](i) transient and [Na+], increased during the pa rtial recovery of contraction in the presence of D2O. We conclude that a de crease in the myofilament sensitivity for Ca2+ and a reduction in Ca2+ infl ux: via I-Ca are principally responsible for the negative inotropic effect of D2O in cardiac muscle. We found no evidence to explain the negative inot ropic effect of D2O in terms of microtubule proliferation. In addition we s uggest that acute application of D2O is not a useful procedure for the inve stigation of the role of microtubules in excitation-contraction coupling in cardiac muscle.