EFFECTS OF TRIMETAZIDINE ON PH(I) REGULATION IN THE RAT ISOLATED VENTRICULAR MYOCYTE

1 We have examined the effects of trimetazidine (TMZ) on intracellular pH (pH(i)) regulation in rat isolated ventricular myocytes. pH(i) was recorded ratiometrically by use of the pH-sensitive fluoroprobe, carb oxy-SNARF-1 (carboxy-seminaphtorhodafluor). 2 Following an intracellul ar acid load (induced by 10 mM NH4Cl removal), pH(i) recovery in HEPES -buffered Tyrode solution was significantly slowed down upon applicati on of 0.3 mM TMZ only when myocytes were pretreated for 5 h 30 min (sl owing by similar to 50%; P < 0.01). This effect of TMZ on pH(i) recove ry was shown to be not only time- but also dose-dependent with a large , quickly reversible, effect obtained with 1 mM TMZ applied for 2-3 h (slowing by similar to 64%; P < 0.001). This slowing of pH(i) recovery was also associated with a decrease of the NH4+ removal-induced acidi fication. 3 Relationship between intracellular intrinsic buffering pow er (beta(i)) and pH(i) was assessed in absence or presence of TMZ (0.3 mM or 1 mM). As expected, beta(i) increased roughly linearly with a d ecrease in pH(i) in all cases. However, both concentrations of TMZ sig nificantly increased beta(i) (by similar to 55 and 65% at pH(i) 7.1, r espectively). 4 When Na+/H+ exchange was inhibited by dimethyl amilori de (DMA; 40 mu M), trimetazidine (1 mM) did not change the H+ flux est imated at pH(i) 7.1 (0.31 +/- 0.03 mequiv 1(-1) min(-1), n = 5, contro l, versus 0.30 +/- 0.025 mequiv 1(-1) min(-1), n = 5, TMZ), ruling out any effect of TMZ on background acid loading.5 Acid efflux carried by Na+/H+ exchange was significantly decreased only when myocytes were p retreated with 1 mM TMZ, for 2-3 h (J(H)(e) = 2.86 +/- 0.38 mequiv 1(- 1) min(-1), n = 26, control, versus 1.66 +/- 0.26 mequiv 1(-1) min(-1) , n = 10, TMZ, estimated at pH(i) 7.1; P < 0.05). 6 In conclusion, the present work demonstrates that, following an intracellular acid load in HEPES-buffered medium, trimetazidine slows down pH(i) recovery in r at isolated ventricular myocytes, primarily through an increase of bet a(i). An effect on Na+/H+ exchange is also detected but only after lon g-term incubation of the myocytes with TMZ.