Regional differences in the role of hyperpolarization-activated current "if" in pacemaker potential within the intact sinoatrial node of the heart in rabbit

ABSTRACT

The sinoatrial [SA] node is not a uniform tissue in terms of its histology and electrophysiology. The regional differences in action potential configuration are well known, for which the difference on ionic currents underlying the regional differences in electrical activity is the most probable cause. The ionic current "if" is though to play a major role in SA node for pace maker slope of action potential. This predication was tested in the present study by block of the current on pacemaker activity in the periphery and center of the intact SA node of the rabbit heart by means of 2mM Cs[+] for 20 min. First, the action potential of different regions of SA node was recorded by conventional glass microelectrode without Cs[+] and nine criteria of action potential configuration were measured. Then, Cs[+] was used and again criteria were measured. In all cases, Cs[+] significantly decreased the pacemaker slope [PS] from 20 +/- 1 [control] to 4 +/- 1mV/s [n=8] in peripheral zone, from 32 +/- 4 [control] to 17 +/- 2mV/s [n=7] in transitional zone and from 49 +/- 3 [control to 36 +/- 4mV/s[n=6] in central zone. Meanwhile, 2mM Cs[+] also decreased the rate of spontaneous activity by 12 +/- 1%. Instead of rate, the cycle length [CL] was recorded, where the increment on cycle length was reported to be 14 +/- 1%."if" current plays a major role in the pacemaker activity in central zone rather than the peripheral zone of intact SA node of the rabbit heart