Extracellular Na removal enhances granule secretion in platelets 3-evidence that Na/H exchange is inhibitory to secretion induced by some agonists

The effect of extracellular Na⁺ (Na⁺]_e) removal on agonist-induced granule secretion in platelets in relation to ph]_i and Ca²⁺]_i changes was investigated. Substitution of Na⁺]_e with choline⁺ of K⁺ resulted in a significant enhancement of 5HT secretion induced by thrombin, collagen, U46619 and the protein kinase C activators, PMA and diC₈. Increases in Ca²⁺]_i induced by thrombin and U46619 were slightly inhibited or unaffected in these buffers, but pH]_i increases induced by thrombin, U46619, PMA and diC₈ were abolished and a drop in pH]_i (0.05–0.1 units below resting) was observed. Although preincubation with potassium acetate produced a big drop in pH]_i and greatly increased secretion with all the agonists, particularly in the absence of Na⁺]_e, clear evidence that pH]_i rises due to Na⁺/H⁺ exchange are inhibitory to secretion was obtained only with thrombin. Thus, (i) NH₄Cl, which restored the increase in pH]_i in the absence of Na⁺]_e reduced the potentiated secretory response to thrombin, (ii) no increase in thrombin-induced secretion was observed when Na⁺ was replaced with Li⁺, which allowed a normal increase in pH]_i and (iii) ethyl isopropyl amiloride (EIPA) abolished the pH]_i rise and potentiated thrombin-induced secretion. With collagen and U46619, the results suggest that removal of Na⁺]_e per se rather than inhibition of Na⁺/H⁺ exchange results in enhanced secretion. It is concluded that Na⁺]_e per se and pH]_i elevations via Na⁺/H⁺ exchange both have important inhibitory roles in the control of platelet granule secretion.