The effects of [K+](o) on regional differences in electrical characteristics of ventricular myocytes in guinea-pig

Altering [K+](o) might have different effects on action potential duration (APD) in myocytes from different regions. Therefore, the effects of [K+](o) on regional differences in action potential characteristics were investiga ted in sub-endocardial, mid-myocardial and sub-epicardial myocytes isolated from the base of guinea-pig left ventricular Free wall using three differe nt [K+](o) (2.7, 5.4 and 8.1 mM KCl). Action potentials were recorded using the switch-clamp technique at 0.5 Hz. Increasing [K+](o) from 2.7 to 8.1 m M shortened the action potential duration to 90 % repolarization (APD(90); mean APD(90) values in sub-endocardial, mid-myocardial and sub-epicardial m yocytes were, respectively, 295 +/- 9, 286 +/- 9 and 266 +/- 8 ms in 2.7 mn l [K+](o), 270 +/- 7, 255 +/- 7 and 215 +/- 7 ms in 5.4 mM [K+](o), 234 +/- 7, 212 +/- 10 and 155 +/- 8 ms in 8.1 mM [K+](o)), depolarized the resting potential, and reduced the amplitude of the action potential. The effect o f increasing [K+](o) on action potential characteristics was more pronounce d in sub-epicardial myocytes than in subendocardial and mid-myocardial myoc ytes. The regional differences in APD(90) in 5.4 mM [K+](o) were increased in 8.1 mM [K+](o) and abolished in 2.7 mM [K+](o). In conclusion, changing [K+](o) produces more pronounced effects on action potentials in sub-epicar dial myocytes than in sub-endocardial myocytes, modifying the normal hetero geneity of action potentials. The differences in the response of sub-epicar dium and sub-endocardium to [K+](o) may contribute to the flattening or inv ersion of the T wave commonly seen in patients presenting with hypokalaemia and the upright and tall T waves observed in electrocardiograms recorded d uring hyperkalaemia, although the underlying ionic currents remain to be de termined.