The Effect of Isoproterenol on the Turtle Ventricular Strips In this study, we have investigated the effect of isoproterenol on the ventricular strip of turtle prepared according to our method. This was described earlier. Briefly, strips about 3-4 cm long and 0.3 cm wide was obtained by cutting horizontally ventricular muscle. Strips thus prepared showed no spontaneous activity but were able to be electrically driven. The bath contained Ringer solution (NaCI 110, KCI 2.7, CaCL 1.04, NaH.PO. 0.06, MgCU 0.1, NaHCOa 2.4 and glucose 11 mM) and oxygenated. The temparature was maintained at 30°C. The strip was placed between a pair of electrodes embedded in-perspex, connected to stimulator which will deliver rectangular pulses of 1 - 2 msc duration. The other stimulation conditions were 9 mV and 0.75 Hrz. Strips showed regular and consistent responses. Isotonic contraction were recorded by an isotonic lever which magnified them about 7 times on smoked drum. Tissue was equilibrated for about 1.5-2 hours. Isoproterenol (10—11, 10—10, 10~~9 and 10—8 M) was added to the bath medium by using cumula¬tive dose tecnique. First addition generally produced dose dependent positive inotropic effect. Tachyphylaxis to this effect was observed in the 2nd, 3rd and 4th aplications. Some strips showed negative inotropic responses after first cumulative addition. The same strips were also used in the second and third day. In spite of storing the strips one or two night in the refrigerator about at + 10°C, they survived and responded again to the drug. Tachyphylaxis was again observed. Pretreatment with practolol ( 3.78 x 10—(i or 3.78 x 10—5 M ) during the equilibrium period did not significantly alter the sensitivity to the drug. Tachyphylaxis reappeared. Tissue was able to respond to the drug during second day.