Effects of arginase inhibition on myocardial Ca²⁺ and contractile responses

Nitric oxide (NO) is thought to increase cardiac contractility by increasing cytosolic Ca²⁺ concentration ([Ca²⁺ ]_cyt ) during excitation. Alternatively, NO could increase the sensitivity of the contractile response to [Ca²⁺ ]_cyt (Ca²⁺ sensitivity). Arginase regulates NO production by competing with NO synthase (NOS), and thus, arginase inhibition should increase cardiac contractility by increasing NO production. We hypothesized that arginase inhibition increases cardiac contractility by increasing both [Ca²⁺ ]_cyt and Ca²⁺ sensitivity. [Ca²⁺ ]_cyt and contractile (sarcomere length [SL] shortening) responses to electrical stimulation were measured simultaneously in isolated rat cardiomyocytes using an IonOptix system. In the same cardiomyocytes, measurements were obtained at baseline, following 3-min exposure to an arginase inhibitor (S-[2-boronoethyl]-l-cysteine; BEC) and following 3-min exposure to BEC plus a NOS inhibitor (N^G -nitro-l-arginine-methyl ester; l-NAME). These responses were compared to time-matched control cardiomyocytes that were untreated. Compared to baseline, BEC increased the amplitude and the total amount of evoked [Ca²⁺ ]_cyt , and the extent and velocity of SL shortening in cardiomyocytes, whereas addition of l-NAME mitigated these effects. The [Ca²⁺ ]_cyt at 50% contraction and relaxation were not different across treatment groups indicating no effect of BEC on Ca²⁺ sensitivity. The [Ca²⁺ ]_cyt and SL shortening responses in time-matched controls did not vary with time. Arginase inhibition by BEC significantly increased the amplitude and the total amount of evoked [Ca²⁺ ]_cyt , and the extent and velocity of SL shortening in cardiomyocytes, but did not affect Ca²⁺ sensitivity. These effects of BEC were mitigated by l-NAME. Together, these results indicate an effect of NO on [Ca²⁺ ]_cyt responses that then increase the contractile response of cardiomyocytes.