How does decreasing the temperature of the Ringer's solution affect the heart rate of a frog?

Decreasing the temperature of the Ringer's solution has a definitive impact on the heart rate of a frog, leading to a decrease in heart rate. This phenomenon is primarily due to the physiological effects of temperature on enzyme activity and metabolic processes.

As temperature falls, the metabolic rate of cardiac cells decreases, leading to slower rates of depolarization and repolarization of cardiac muscle. This reduction in metabolic activity means that the heart's pacemaker cells (the sinoatrial node) generate action potentials at a slower rate, ultimately resulting in a lower heart rate. The heart in ectothermic animals, such as frogs, is particularly sensitive to changes in temperature, demonstrating significant variations in heart rate in response to thermal alterations.

In contrast, a higher temperature generally speeds up metabolic processes and enhances heart activity, while an intermediate temperature allows for a more stable heart rate. Therefore, the decrease in temperature is associated directly with a decrease in heart rate, thus confirming the correctness of the selection indicating decreased heart rate as the response to lower temperatures in Ringer's solution.