Resting Heart Rate, Drugs & Ligature on Cardiac Muscle

The northern leopard frog (Lithobates pipiens) was cooled in ice water for 15 minutes and then double pithed. The skin over the pectoral girdle and pectoral girdle itself was cut and removed to uncover the frog’s beating heart. The area was flushed with Ringer’s solution. A metal hook with a thread was inserted into the apex of the ventricle. The loose end of the thread was tied into the hole of the blade of the transducer. The transducer was raised until the heart was raised out of the cavity of the frog and the thread was taut. The heart was then ready for testing.

In order to test the hypothesis we used a force transducer, suture thread and LabScribe software. The frog was positioned so that heart was directly below the end of the transducer connected to a ring stand. The suture thread was tied taut to the forced transducer at a 90 degree angle. Cardiac muscle was bathed periodically in Ringer’s solution.

The first experiment was conducted to measure the mechanical traced produced by the contraction of a resting heart and to determine the resting heart rate. Using the software, heart contractions were measured for fifteen seconds.

The second experiment measured the effects of cold temperature on heart rate. Ten drops of room temperature Ringer’s solution were applied to the heart. After twenty seconds five drops of chilled Ringer’s solution were applied to the heart. Measurements were recorded until the heart recovered.

The third experiment tested the effects of Epinephrine and Acetylcholine on the amplitude and rate of the heart contraction. Two drops of room temperature Epinephrine solution was applied to the heart and contractions were recorded for sixty seconds. The heart was rinsed with Ringer’s solution until the heart returned to the resting rate. One drop of room temperature Acetylcholine solution was applied to the heart and heart contractions were measured for sixty seconds followed by rinsing the heart with Ringer’s solution until the heart returned to the resting rate. Two drops of room temperature atropine was applied to heart and contractions were measured for thirty seconds then one drop of Acetylcholine was applied. Measurements were recorded for sixty seconds followed with bathing the heart with Ringer’s solution until the heart returned to the resting heart rate.

The last experiment measured the effects of using a ligature to isolate the ventricle from the SA node and interrupt the communication between the atria and the ventricle. A piece of thread was tied in a single hand knot around the AV groove that separates the atria and ventricles. The knot was successively tightened until the atria and ventricle contracted independently.