Sex-dependent, zinc-induced dephosphorylation of phospholamban by tissue-nonspecific alkaline phosphatase in the cardiac sarcomere.

We have previously reported that Zn(2+) infused into the coronary arteries of isolated rat hearts leads to the potent dephosphorylation of phospholamban (PLB) as well as a noticeable but less potent dephosphorylation of the ryanodine receptor 2. We hypothesized in the present study that a Zn(2+)-activated phosphatase is located in the vicinity of the sarcoplasmic reticulum (SR) where PLB and ryanodine receptor 2 reside. We report here the novel finding of tissue-nonspecific alkaline phosphatase (TNAP), a zinc-dependent enzyme, localized to the SR in the cardiac sarcomere of mouse myocardium. TNAP activity was enhanced by injection of Zn acetate into a tail vein before harvesting the heart and imaged using electron microscopy of electron dense deposits indicative of the hydrolysis of exogenous β-glycerophosphate. TNAP activity was observed localized to the ends of the Z-line corresponding to SR and was qualitatively more visible in myocardium of males compared with females. Correspondingly, PLB phosphorylation status was potently reduced in myocardium of males injected with Zn acetate, whereas there was no apparent effect of Zn acetate injection on PLB phosphorylation in females. Surprisingly, Western blot analysis of TNAP content suggested a significantly lower TNAP content in males compared with females. These data suggest that TNAP plays a role in governing the phosphorylation status of calcium handling proteins in the SR. Furthermore, the content and activity of TNAP are differentially regulated between the sexes and thus may account for some sex differences in cardiopathologies associated with calcium handling.