Effects of uremic serum on isolated cardiac myocyte calcium cycling and contractile function

Background. Diastolic dysfunction occurs in patients with chronic renal fai lure. Moreover, serum from uremic patients contains one or more inhibitors of the plasmalemmal Na,K-ATPase (sodium pump). We hypothesized that a circu lating substance present in uremic sera contributes to both sodium pump inh ibition and diastolic dysfunction. Methods. Serum samples were obtained from six patients with chronic renal f ailure and diastolic dysfunction. Results. Their serum samples caused marked inhibition of Na,K-ATPase purifi ed from dog kidney at all concentrations studied (all P < 0.01) and also im paired ouabain-sensitive rubidium uptake by myocytes isolated from Sprague- Dawley rats (P < 0.01). These cardiac myocytes were studied for their contr actile function with video-edge detection and calcium metabolism with indo- 1 fluorescence spectroscopy after exposure to these uremic sera. These urem ic sera caused increases in myocyte fractional shortening (P < 0.01) as wel l as an increase in the time constant of relengthening (P < 0.01). Examinin g the calcium transient, the time constant for calcium recovery was also in creased (P < 0.01). Exposure of these cells to sera from age- and sex-match ed healthy subjects did not result in significant changes in contraction or calcium cycling. Extracts of uremic serum samples inhibited isolated Na,K- ATPase whereas extracts of normal serum samples did not. The effect of urem ic serum extracts on contractile function and calcium cycling were quite si milar to that of intact serum or the addition of ouabain. Co-incubation of uremic serum extract with an antibody fragment directed against digoxin mar kedly attenuated the inhibition of Na,K-ATPase activity and completely prev ented any effects on calcium cycling or contractile function. Conclusion. These data show that one or more substances are present in urem ic sera that acutely cause increased force of contraction and impaired reco very of cardiac myocyte calcium concentration as well as impaired relaxatio n. As these effects are similar to that seen with ouabain and can be preven ted by co-incubation with an antibody fragment to digitalis, which also att enuates the sodium pump inhibitory effect, we suggest that this (these) sub stance(s) circulating in uremic sera and inhibiting the sodium pump also ca uses the acute diastolic dysfunction seen in our system.