Lack of effect of neuraminidase on responses of isolated guinea-pig heart preparations to ouabain

Inotropic effects of Ca2+ channel agonist and antagonists in neuraminidase-treated left atria of rats

The effects of removal of sialic acid from cardiac sarcolemma on contractile functions and on inotropic responses to Ca2+ channel agonist and antagonists were investigated in rat left atria. About 64% of the total sialic acid content of the left atria was removed during a 90 min exposure to neuraminidase (2 u ml-1). The removal of sialic acid neither affected the development of twitch tension induced by stimulation at a frequency of 0.5 Hz, nor altered the interval-dependent changes in contractility such as the force-frequency relationship and post rest contractions. The positive inotropic effects produced by isoprenaline, and by an increase in extracellular Ca2+ concentration were the same in the neuraminidase-treated preparations as those in the untreated preparations. Bay K 8644, a Ca2+ channel agonist, induced an increase in contractility in the neuraminidase-treated preparations comparable to that in the untreated ones. Neuraminidase treatment significantly attenuated the negative inotropic effects of verapamil and diltiazem, whereas it had no effect on that of nifedipine. The results indicate that sialic acid removal modifies neither the basal contractile functions nor the positive inotropism which is associated with an enhancement of the slow inward Ca2+ current. However, sialic acid, which constitutes the glycocalyx of the cardiac sarcolemma, may be involved in the mechanism of the Ca2+ channel antagonistic actions of verapamil and diltiazem, but not that of nifedipine. Thus, our results provide pharmacological evidence that verapamil and diltiazem behave differently from the dihydropyridine compounds.

Effect of neuraminidase treatment on the inotropic response to ouabain, isoproterenol and calcium in the guinea pig heart

To determine the role of the glycocalyx sialic acids residues in excitation-contraction coupling and the inotropic response to cardiotonic agents, we studied the effect of neuraminidase treatment on the response to ouabain, isoproterenol, calcium and reduced extracellular sodium in Langendorff preparations of adult guinea pig hearts. Neuraminidase treatment (0.01 unit/ml, 1 h) reduced the magnitude of the positive inotropic response to 2.5 X 10(-7) M ouabain and the maximum response to 5 X 10(-7) M ouabain by about 46% and 30%, respectively, but did not prevent ouabain toxicity. Neuraminidase treatment did not affect the contractility produced by calcium concentration alterations up to 5 mM calcium or the positive inotropic effect produced by lowering external sodium to as low as 80 mM. The inotropic response to as high as 10(-8) M isoproterenol was also not affected. The contractility response developed to calcium concentrations greater than 5 mM and to 5 X 10(-8) M isoproterenol were significantly reduced (P less than 0.05) by neuraminidase treatment. The content of sialic acids in neuraminidase-treated hearts used in the above concentration-response studies of ouabain, isoproterenol, calcium, and sodium was reduced by 70.7%, 66.1%, 65.6% and 66.2%, respectively. Neuraminidase treatment had no effect on basal (Na+ - K+)ATPase and Mg2+ - ATPase activities of (Na+ - K+)ATPase-containing membrane preparations of the guinea pig left ventricle. Neuraminidase treatment neither influenced the sensitivity of the enzyme (Na+ - K+)ATPase to ouabain inhibition nor did it affect the characteristics of [3H]ouabain binding to the preparation. These results suggest that the sialic acids of the glycocalyx in the guinea pig left ventricle play an important role in part of the inotropic response to subtoxic concentrations of ouabain.