High calcium can be seen that the contraction force of frog heart is enhanced, but the relaxation is incomplete, which makes the contraction baseline move up. When the concentration of calcium ion is high, the heart will stop in the contraction state, which is called "calcium rigidity". The contraction activity of myocardium is related to the concentration of calcium ions in myocardial sarcoplasmic fluid. Myocardial sarcoplasmic reticulum is underdeveloped, and its calcium storage capacity is poor, which is easily affected by extracellular calcium ion concentration. When the concentration of calcium ion rises to 10-5m, troponin, as a calcium receptor, binds enough calcium ion to cause the change of molecular configuration of troponin, thus triggering myofilament sliding and muscle fiber contraction. When the concentration of calcium ion in muscle serous fluid drops to 10-7m, calcium ion dissociates from troponin and the myocardium relaxes. Perfusion of frog heart with high calcium renzi solution makes the concentration of calcium ions in the sarcoplasm increase continuously, and the number of calcium ions bound to troponin increases continuously, even to the extent of only binding without separation, so calcium rigidity appears in the myocardium.

After dripping epinephrine, it can be seen that the frog's heart contraction is enhanced, the heart relaxation is complete, and the amplitude of heartbeat curve is obviously increased. Because adrenaline enhances myocardial contractility. The mechanism is that adrenaline binds to β receptor on myocardial cell membrane, which improves the permeability of calcium ion in myocardial cells and sarcoplasmic reticulum, leads to the increase of calcium ion concentration in sarcoplasmic fluid and enhances myocardial contraction. In addition, epinephrine can also reduce the affinity of troponin with calcium ions and promote the release rate of troponin to calcium ions. Increase the uptake rate of calcium ions by sarcoplasmic reticulum, stimulate the exchange of sodium and calcium ions, and accelerate the discharge of intracellular calcium ions during repolarization. In this way, the myocardial relaxation speed is accelerated and the whole relaxation process is obviously strengthened.

After dropping acetylcholine, the contraction of frog heart weakened, the baseline of contraction curve moved down and the heart rate slowed down. Finally, the heartbeat stops in diastolic phase, which is similar to the change of high potassium. Because acetylcholine can weaken the contractility of myocardium. The mechanism is that acetylcholine binds to M receptor in myocardial cells. On the one hand, it improves the permeability of potassium channels in myocardial cell membrane and promotes the outflow of potassium ions, which will increase the outflow of potassium ions during the repolarization of sinoatrial node cells (1) and increase the absolute value of maximum repolarization potential. The attenuation process of ik is weakened and the speed of automatic depolarization is slowed down. These two factors lead to the decrease of sinoatrial node autonomy and slow heart rate. (2) During repolarization, the outflow of potassium ions increased, the action potential phases 2 and 3 shortened, and the calcium ions entering the cells decreased, which weakened the myocardial contractility; On the other hand, acetylcholine can directly inhibit calcium channels, reduce the influx of calcium ions and weaken the contraction of myocardial cells.

2. Staging and mechanism of ventricular action potential?

(1)0 phase (depolarization process)

The fast sodium channel is activated and a large number of sodium ions flow in.

(2) 1 phase (initial stage of rapid repolarization)

Potassium ion outflow

(3) The second stage (platform period)

Calcium ion inflow and potassium ion outflow are in a relatively balanced state.

(4) Stage 3 (end of rapid repolarization)

When the L-type calcium channel is deactivated and closed, the inward flow of calcium ions stops, while the outward flow of potassium ions gradually increases.

(5) The fourth stage (recovery period)