PRIMAQUINE, AN INHIBITOR OF VESICULAR TRANSPORT, BLOCKS THE CALCIUM-RELEASE-ACTIVATED CURRENT IN RAT MEGAKARYOCYTES

The whole-cell patch-clamp technique was used to study the effect of p rimaquine, an inhibitor of vesicular transport, on the calcium-release -activated current (I-erac) in rat megakaryocytes. Addition of primaqu ine, before emptying of internal Ca2+ stores by ionomycin, prevented t he development of I-erac, with a half-maximal concentration of near 10 0 mu M. Maximal inhibition (greater than or equal to 83 %) was observe d at 0.6-1 mM primaquine. At 1 mM, chloroquine, a related compound whi ch is less effective at blocking vesicular secretion, had no effect on I-erac. Primaquine (0.8 mM) added after sustained activation of I-era c caused a gradual block of current, with maximal inhibition of 50 % o bserved after 2-3 min. At 1 mM, internal guanosine 5'-[gamma-thio]trip hosphate reduced I-erac by 65+/-13 %. Neither 1 mM GTP nor 2 mM guanos ine 5'-[beta-thio]diphosphate had any significant effect on I-erac. Th e recognized role of GTPases in the regulation of vesicular traffickin g, together with block of I-erac activation by primaquine, provide evi dence that the channels carrying I-erac may be stored in a vesicular m embrane compartment and transferred to the plasma membrane following s tore depletion.