[ effect of block of the ultrarapid delayed rectifier potassium current [Ikur] by 50 micro M 4-amino pyrimidine on pacemaker activity in the mouse sinoatrial and atriovetricular nodes]

In the heart cells, ultrarapid delayed rectifier potassium current [IKur] is one of the important currents in action potential repolarization phase. Ultrarapid delayed rectifier potassium current is specifically blocked by low concentration of 4-Amino Pyrimidine [4-AP]. The aim of this study was to determine the effect of low concentration of 4-Amino Pyrimidine blocker on pacemaker activity of sinoatrial node [SAN] and atrioventricular node [AVN] of mouse heart. SAN and AVN were separated and the pacemaker activity of distinct intact SAN and AVN was recorded before and during 50 micro M 4-AP by two separate metal microelectrodes that were in contact with the endothelail surface of the nodes. Then the action potential cycle length [CL] was measured. 50 micro M 4-Amino Pyrimidine increased the action potential cycle length of SAN and AVN preparations respectively by 20.2 +/- 3.3% and 18 +/- 3%. These increases on the action potential cycle length were significant in both nodes. According to the results, IKur is present in both SAN and AVN nodes and the effect of 50 micro M 4-AP on action potential cycle length [CL] of the two nodes is the same

[Comparing the effect of neuronal type Na+channel block on pacemaker activity of C57BL6/J mouse sinoatrial and atrioventricular node]

In resent years, neuronal type Na+ channel is one of the important currents for action potential depolarization phase in heart cells. In this study neuronal type Na+ channel is blocked by low concentration of TTX to compare the effect of TTX blocker on pacemaker activity of sinoatrial node [SAN] and atrioventricular node [AVN] of mouse heart. In this experimental study the pacemaker activity of distinct intact SAN and AVN, was recorded before and during consuming 100 nM TTX and cycle length [CL] was measured. Data was analyzed using T test. 100 nM TTX increased CL on SAN preparations by 22.2 +/- 6% and on AVN preparations by 52.5 +/- 13.5%. These changes were significant in the two nodes. It is passible to conclude that; the neuronal type Na+ channel was present in the two nodes, and the effect of TTX on CL of the two nodes was different

[Investigating the action of ryanodine receptor on cycle length of action potential of the rabbit intact sinoatrial and atrioventricular nodes]

The role of ryanodine canal/receptor current on the pacemaker activity of heart cells is controversial. This study was performed to characterize the effects of ryanodine receptor [RyR], on the cycle length [CL] of action potential of the sinoatrial node [SAN] and the atrioventricular node [AVN] of rabbit heart. As a specific blocker of the RyR, the effects of 0.2 and 2 micromolar of Ryanodine on action potential CL was investigated. The action potential was recorded and measured from endothelial surface of intact SAN and AVN by two separated metal microelectrode before and after using ryanodine. The experimental method used, was intervention. Data analysis was performed using independent and paired T test. Inhibition of RyR by 0.2 and 2 micro M of Ryanodine prolonged CL by 11.6 +/- 4.3 and 30 +/- 6.75% in SAN preparations and by 18.5 +/- 6.3 and 65 +/- 11% in AVN preparations. This effect was significantly [p < 0.05] longer on AVN than SAN. Rabbit SAN and AVN showed distinct different responses to the inhibition of RyR, which reflects the variation in contribution of this current to the pacemaker function of the cardiac nodes in the rabbit. As ryanodine did not cease pacemaker activity of any tissue preparation for two nodes; therefore, it is possible to say that ryanodine receptor has no absolute role in generating action potential of nodes

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

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