In order to investigate the regional electrophysiological characteristics of the rabbit SA node, the node was tied with fine silk threads into small preparations (0. 5 x 0. 5 mm in size) and electrotonic connection between them was insulated. The preparations were divided into three groups, as peripheral, transitional and central areas. Three glass microelectrodes were inserted into the cells in each area and the data were collected when transmembrane potentials of each area was recorded simultaneously.
1. As the Na+ concentration of the solution was reduced to 80, 60, and 405Vo of control, the beating frequency, the maximum diastolic potential (MDP) , overshoot and the maximum upstroke velocity (Vmas) were decreased. The decrease of the beating frequency was more marked in the central area, and the spontaneous besting was arrested by the reduction of Na´ concentration more than 600:
2. In the peripheral area, MDP was decreased as the 2mM KI concentration reduced to 1 mM. At high K} concentration (12 mM), the spontaneous beating of the central area was stopped in the 500 of preparations.
3. The decrease of the spontaenous beating frequency, due to Ach(Acetylcholine) was more prominent in the central area. The increase of the MDP was marked in the central area, however, the decrease of overshoot was distinct in the peripheral area.
4. The peripheral area was the most sensitive to TTX(Tetrodotoxin). The effects of epinephrine which changed the parameters of the action potential were more marked in the central area, but the effects were more marked in the peripheral area when the preparations were pre-treated with TTX.
5. The amplitude of the-Na+-K+ pump activity of the peripheral area was larger and decreased in the transitional and the central area in sequence.
The electrophy siological characteristics of the SA nodal cells have shown various differences by the locations. Especially, the central area which considered as the leading pacemaker are generally more sensitive to the various interventions
|