Leptin regulates cardiovascular function. Leptin levels are elevated in obesity and hypertension and may play a role in cardiovascular dysfunctions in these comorbidities. This study was designed to determine the influence of hypertension on the cardiac contractile response of leptin. Mechanical and intracellular Ca(2+) properties were evaluated using an IonOptix system in ventricular myocytes from spontaneously hypertensive (SHR) and age-matched Wistar Kyoto (WKY) rats. The contractile properties included peak shortening (PS), duration and maximal velocity of shortening/relengthening (TPS/TR(90), +/-dL/dt), and fura-fluorescence intensity change (DeltaFFI). NO and nitric oxide synthase (NOS) activity were assessed by the Griess and the (3)H-arginine/citrulline conversion assays, respectively. The leptin receptor (Ob-R) and the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway were evaluated by Western blot analysis. SHR animals displayed significantly elevated blood pressure and plasma leptin levels. Leptin elicited a concentration-dependent inhibition of PS and DeltaFFI in WKY, but not in SHR myocytes. Leptin did not affect TPS, TR(90), or +/- dL/dt. The difference in leptin-induced contractile response between the WKY and the SHR groups was abolished by the NOS inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME), but not by elevated extracellular Ca(2+). Either the JAK2 inhibitor AG-490 or the mitogen-activated protein (MAP) kinase inhibitor SB203580 abrogated the leptin-induced response in the WKY myocytes, whereas AG-490 unmasked a negative response in PS in the SHR myocytes. SHR myocytes displayed similar Ob-R protein abundance and basal NO levels, a blunted leptin-induced increase in NOS activity as well as enhanced basal STAT3 levels compared with the WKY group. These data indicate that the leptin-induced cardiac contractile response is abolished by spontaneous hypertension, possibly because of mechanisms involving altered JAK/STAT, MAP kinase signaling, and NO response.