Serum ALT: tarnished gold standard for interferon response in hepatitis C virus infection.
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Alanine Transaminase↗
Biomedical subjects
H F Yee
Publications and source records attributed to H F Yee.
Effects of neuraminidase on cellular calcium and contraction in cultured cardiac myocytes.
Mammalian plasma membranes, including the myocardial sarcolemma, are abundantly glycosylated. Sialic acid is a ubiquitous anionic sugar found at the periphery of sarcolemmal glycoconjugates. The physiological role of this sugar is not clear, but neuraminidase, which specifically hydrolyzes sialic acid from the sarcolemma, has been found to increase calcium exchange, cause electrophysiological abnormalities, and enhance the transient (T) calcium current in cardiac myocytes. The purpose of this study was to better characterize the effect of neuraminidase on cellular calcium (Ca) and contractile function. Neuraminidase removed up to 57% of total sialic acid from the cells. 45Ca exchange was measured and neuraminidase was found to increase cell calcium proportional to the amount of sialic acid removed (18.6 +/- 0.8 mmol/kg dry w, maximally). Over 80% of the increment in calcium remained rapidly exchangeable (t1/2 less than 15 s) under non-perfusion limited conditions and was inhibited by cations (La greater than Cd greater than Mn greater than Mg) and nifedipine. Using a video-monitoring system, neuraminidase was observed to transiently increase cell shortening during contraction (30 +/- 9%), with progression to arrhythmias followed by cessation of contraction. These results indicate that neuraminidase, probably by removing sarcolemmal sialic acid residues, greatly augments cellular calcium in cultured cardiac myocytes. Most of the increment in Ca induced by neuraminidase was very rapidly exchangeable and most likely mediated by a Ca specific mechanism. Additionally, neuraminidase treatment altered contractile function in a manner consistent with elevated cellular Ca. Despite the many-fold increase in cellular Ca induced by sialic acid removal, cells recovered and demonstrated rhythmic contractions upon return to control incubation conditions.
Animals↗
Neuraminidase selectively enhances transient Ca2+ current in cardiac myocytes.
Sialic acid, an anionic sugar moiety found peripherally on membrane glycoconjugates, is specifically hydrolyzed from the cell surface by neuraminidase. Because neuraminidase has previously been demonstrated to augment myocardial cell calcium content, the effects of neuraminidase on Ca channel function were studied on voltage-clamped guinea pig ventricular myocytes. In 25-50% of cells, neuraminidase treatment (0.12 U/ml for 20 min) enhanced current through the transient (T) Ca channel by 304 +/- 35% without significantly altering the magnitude of the long-lasting (L) Ca channel current. Exposure to neuraminidase did not affect the voltage dependence of activation or inactivation, nor did it affect the selective inhibition of the T-channel current by amiloride or the L-channel current by nifedipine. After neuraminidase treatment, the T-channel current inactivated more rapidly (time constant decreasing from 8.9 +/- 0.9 to 7.7 +/- 0.6 ms), whereas there was no change in the rate of inactivation of the L-channel current. Neuraminidase treatment removed approximately 20% of the total cellular sialic acid. These results indicate that neuraminidase treatment selectively modulates the function of the T Ca channel in ventricular myocytes, possibly through removal of sarcolemmal sialic acid, suggesting that glycosylation of membrane macromolecules may influence membrane function.
Animals↗