Effect of concanavalin A on intracellular calcium concentration in single blood platelets

Mobilization of Ca++ was estimated in single rabbit blood platelets with digital imaging microscopy. Concanavalin A (Con A) caused a rapid initial increase in intracellular concentration of Ca++ ([Ca++]i) with a latent time of about 20 sec, followed by a sustained increase in [Ca++]i. This effect of Con A was antagonized by alpha-methyl-D-mannose, which already was shown to antagonize the inhibitory effect of Con A on 5-HT transport, indicating that this effect of Con A was also derived from its binding to cell surface glycoproteins. The presence of EGTA in the medium did not affect the initial rise, but inhibited the latter phase of sustained rise. Thus, Con A induced elevation of [Ca++]i was suggested to consist of two different processes: mobilization of Ca++ from the intracellular storage sites and successive Ca++ influx through Ca++ channels. The effect of Con A on the 5-HT transport was tested in the presence of EGTA, a condition where no Ca++ influx occurs. The results indicate that Con A induced inhibition of 5-HT transport was not influenced by EGTA in the medium. It is suggested that the effect of Con A on 5-HT transport might be exerted through the Ca++ mobilization from its intracellular storage sites.