Procedure

1. Review a textbook section on regulation of the cardiac cycle.

2. Complete Part A of Laboratory Report 43.

GENERAL SUGGESTION

t ry to become familiar with the content and organization of this lab before you pith a frog. If you work quickly, one pithed frog should last for all of the experimental steps.

3. Observe the normal action of a frog heart. To do this, follow these steps:

a. Obtain a live frog, and pith it according to the directions in Procedure C of Laboratory Exercise 19.

ALTERNATIVE PROCEDURE

a n anesthetizing agent, tricaine methane sulfonate, can be used to prepare frogs for this lab. This procedure eliminates the need to pith frogs.

b. Place the frog in a dissecting tray with its ventral side up, and pin its jaw and legs to the tray with dissecting pins.

Figure 43.1 Pin the frog to the dissecting tray and make incisions through the skin as indicated.

Figure 43.1 Pin the frog to the dissecting tray and make incisions through the skin as indicated.

Figure 43.2 Attach a hook and thread to the tip of the ventricle.

Ventral Side Frog

c. Use scissors to make a midline incision through the skin from the pelvis to the jaw.

d. Cut the skin laterally on each side in the pelvic and pectoral regions, and pin the resulting flaps of skin to the tray (fig. 43.1).

e. Remove the exposed pectoral muscles and the sternum, being careful not to injure the underlying organs.

f. Note the beating heart surrounded by the thin-walled pericardium. Use forceps to lift the pericardium upward, and carefully slit it open with scissors, thus exposing the heart.

g. Flood the heart with frog Ringer's solution, and keep it moist throughout this exercise.

h. Note that the frog heart has only three chambers—two atria and a ventricle. Watch the heart carefully as it beats, and note the sequence of chamber movements during a cardiac cycle.

4. Tie a piece of thread about 45 cm long to a small metal hook, and insert the hook into the tip (apex) of the ventricle without penetrating the chamber (fig. 43.2). The laboratory instructor will demonstrate how to connect the thread to a physiological recording apparatus so that you can record the frog heart movements. The thread should be adjusted so that there is no slack in it, but at the same time it should not be so taut that it pulls the heart out of its normal position (fig. 43 3).

Figure 43.3 Attach the thread from the heart to the recording apparatus so that there is no slack in the thread.

Figure 43.3 Attach the thread from the heart to the recording apparatus so that there is no slack in the thread.

Frogs Heart Process

5. Record the movements of the frog heart for 2-3 minutes. Identify on the recording the smaller atrial contraction waves and the larger ventricular contraction waves. Also, determine the heart rate

(beats per minute) for each minute of recording, 7.

and calculate the average rate. Enter the results in 8.

Part B of the laboratory report.

6. Test the effect of temperature change on the frog's heart rate. To do this, follow these steps:

a. Remove as much as possible of the Ringer's solution from around the heart, using a medicine dropper.

b. Flood the heart with fresh Ringer's solution that has been cooled in an ice water bath to about 10°C (50°F).

c. Record the heart movements, and determine the heart rate as before.

d. Remove the cool liquid from around the heart, and replace it with room temperature Ringer's solution.

e. After the heart is beating at its normal rate again, 9. flood it with Ringer's solution that has been heated on a hot plate to about 35°C (95°F).

f. Record the heart movements, and determine the heart rate as before.

g. Enter the results in Part B of the laboratory report.

Complete Part B of the laboratory report. Test the effect of an increased concentration of calcium ions on the frog heart. If the frog heart from the previous experiment is still beating, replace the fluid around it with room temperature Ringer's solution, and wait until its rate is normal. Otherwise, prepare a fresh specimen, and determine its normal rate as before. To perform the test, follow these steps:

a. Flood the frog heart with 2% calcium chloride. (This solution of calcium chloride will allow ionization to occur providing Ca++.)

b. Record the heartbeat for about 5 minutes and note any change in rate.

c. Flood the heart with fresh Ringer's solution until heart rate returns to normal.

Test the effect of an increased concentration of potassium ions on the frog heart. To do this, follow these steps:

a. Flood the heart with 5% potassium chloride. (This solution of potassium chloride will allow ionization to occur providing K+.)

b. Record the heartbeat for about 5 minutes, and note any change in rate.

10. Complete Part C of the laboratory report.

OPTIONAL ACTIVITY

p lan an experiment to test the effect of some additional factor on the action of a frog heart. For example, you might test the effect of epinephrine, acetylcholine, caffeine, or some other available substance. If the laboratory instructor approves your plan, perform the experiment and record the heart movements. What do you conclude from the results of your experiment?

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