Measurements of intracellular free Ca++

Three putative N-glycosylation sites within the murine 5-HT3A receptor sequence affect plasma membrane targeting, ligand binding, and calcium influx in heterologous mammalian cells

The serotonin type 3(A) receptor (5-HT3(A)R) is a ligand-gated ion channel (LGIC) that modulates a diverse set of cognitive and physiological functions. The 5-HT3(A)R, as with other LGICs, is a pentameric ion channel comprising five glycoprotein subunits. Although the N-terminal of the 5-HT3(A)R contains three putative N-linked glycosylation sites, the importance of each glycosylation site has not yet been established. To address this question, we used tunicamycin treatment and site-directed mutagenesis to inhibit selectively N-linked glycosylation at each site and then examined the effects of these treatments on receptor expression and function in transiently transfected heterologous cells. We show that the murine 5-HT3(A)R is glycosylated and that each N-linked glycosylation site plays a role in receptor regulation. Our findings suggest that N109 is necessary for receptor assembly, whereas N174 and N190 are important for plasma membrane targeting and ligand binding. Furthermore, we demonstrate that each site is necessary for 5-HT3(A)R-mediated Ca(2+) influx. We conclude that N-glycosylation is a critical step in the maturation, trafficking, and function of the murine 5-HT3(A)R.

Stopped-flow kinetics of the interaction of the fluorescent calcium indicator Quin 2 with calcium ions

The kinetics of the Quin 2-Ca2+ interaction have been studied using stopped-flow fluorimetry. Mixing the Quin 2-Ca2+ complex with a large excess of EGTA, EDTA or MgCl2 resulted in first order dissociation kinetics. The observed dissociation rate increased slightly with increasing EGTA concentration yielding a limiting value of 83 +/- 4 s-1 for the dissociation rate constant (k-) at pH 7.2, 37 degrees C, +/- 3mM Mg2+. The temperature dependence of the dissociation was weak (activation energy = 22 +/- 1 kJ/mol) and around neutral pH the pH dependence was negligible. The association reaction was too fast to be monitored directly. From this and the instrument dead-time, the second order rate constant k+ was estimated to be greater than or equal to 10(9) M-1s-1, in agreement with the calculation from k+ = k-/K. These data should be useful in evaluating the potential of Quin 2 to measure fast intracellular Ca2+ transients.