Improvement of defective sarcoplasmic reticulum Ca 2+ transport in diabetic heart of transgenic rats expressing the human kallikrein‐1 gene

The bradykinin‐forming enzyme kallikrein‐1 is expressed in the heart. To examine whether contractile performance and sarcoplasmic reticulum Ca 2+ transport of the diabetic heart can be rescued by targeting the kallikrein‐kinin system, we studied left ventricular function and sarcoplasmic reticular Ca 2+ uptake after induction of streptozotocin‐induced diabetes mellitus in transgenic rats expressing the human tissue kallikrein‐1 gene. Six weeks after a single injection of either streptozotocin (70 mg/kg ip) or vehicle, left ventricular performance was determined using a Millar‐Tip catheter system. The Ca 2+ ‐transporting activity of reticulum‐derived membrane vesicles was determined in left ventricular homogenates as oxalate‐supported 45 Ca 2+ uptake. Western blot analysis was used to quantify the reticular Ca 2+ ‐ATPase SERCA2a, phospholamban, and the phosphorylation status of the latter. Contractile performance and Ca 2+ uptake activity were similar in nondiabetic wild‐type and transgenic rats. Severely diabetic wild‐type animals exhibited impaired left ventricular performance and decreased reticular Ca 2+ uptake (‐39% vs. wild‐type rats, P <0.05, respectively). These changes were attenuated in diabetic transgenic rats that, in addition, exhibited a markedly increased phospholamban phosphorylation at the Ca 2+ /calmodulin kinase‐specific site threonine 17 (2.2‐fold vs. diabetic wild‐type rats, P <0.05). These transgene‐related effects were abolished after treatment with the bradykinin B 2 receptor antagonist icatibant (Hoe 140). The SERCA2‐to‐phospholamban ratio, phosphoserine 16 ‐phospholamban levels, and the apparent affinity for Ca 2+ of the uptake reaction did not differ between the groups. Increasing the activity of the kallikrein‐kinin system by expressing a human kallikrein‐1 transgene protects rat heart against diabetes‐induced contractile and reticular Ca 2+ transport dysfunctions. An increased phosphorylation of the SERCA2 regulatory protein phospholamban at threonine 17 via a B 2 receptor‐mediated mechanism is thereby involved.