Effects of Vanadate on Biochemical and Contractile Properties of Rabbit Hearts

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Abstract
Perfusion of working rabbit hearts with 300 μUM vanadate caused a 30% increase in left ventricular pressure (dp/dt max) and right ventricular wall force (dF/dt) with no change in negative dp/dt max or dF/dt max. Troponin I (TnI) a substrate for cyclic adenosine monophosphate dependent protein kinase, was isolated by affinity chromatography from hearts freeze-clamped at the peak of the inotropic response. The covalent phosphate content of TnI fell from 1.2 moles/mole in control hearts to 0.8 mole/mole in hearts perfused with vanadate. We also isolated myofibrils, microsomes enriched in vesicles derived from the sarcoplasmic reticulum (SR), and mitochondria, and determined the influence of vanadate on their Ca2+-ATPase and, in the case of SR vesicles, on calcium transport. While the ATPase and respiratory activity of mitochondria were unaffected by up to 100 μUM vanadate, SR ATPase and rate of calcium transport measured with 5 mM oxalate and myofibrillar ATPase were inhibited by vanadate with a similar Ki of 50 to 70 μUM. SR ATPase measured without oxalate was more sensitive to inhibition by vanadate (Ki = 5 μUM). Moreover, steady-state accumulation of Ca2+ by SR vesicles measured in the absence of oxalate was not affected by up to 60 μUM vanadate and inhibited by only 11% at 120 μUM vanadate. Our data indicate that the inotropic effects of vanadate may involve sites of action on myofibrils and SR and that vanadate may be a useful tool in the study of ATPase mechanisms.

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