EFFECT OF PLATELET-ACTIVATING-FACTOR ON INTRACELLULAR FREE CALCIUM INCOW TRACHEAL EPITHELIUM

The effect of platelet activating factor (PAF) on the intracellular cy tosolic levels of free calcium ([Ca2+](i)) was studied in cultured epi thelium from cow trachea. In fura-2-loaded cells, PAF (10(-9) to 10(-5 ) M), but not lyso-PAF, increased [Ca2+](i) in a concentration-depende nt manner, from 106 +/- 15 to 270 +/- 40 nM (P < 0.05). This [Ca2+](i) response consisted of a transient increase that peaked within 15 s af ter addition and a subsequent sustained elevation that reached a plate au after 1 min. The potency for the sustained response was greater by approximately 1 log U than that for the transient response. Preincubat ion of the cells with the PAF receptor antagonist CV6209 (10(-6) M) in hibited the increase in [Ca2+](i). Ca2+-free medium (2 mM EGTA) totall y abolished the sustained response to PAF, but it only partially inhib ited the transient response. Verapamil (10(-5) M) also largely inhibit ed the sustained response. Moreover, PAF transiently increased inosito l triphosphate (IP3) levels, peaking 10 s after addition. These data s uggest that (1) a PAF-induced increase in [Ca2+](i) is mediated via PA F receptors, (2) PAF causes both transient [Ca2+](i) release from intr acellular stores through IP3 production and sustained [Ca2+](i) influx from extracellular solution, and (3) Ca2+ influx may be a major pathw ay of PAF-induced increase in [Ca2+](i) in cow tracheal epithelium.