How does temperature affect the heart rate of an ENDOTHERMIC organism?

Answer 1

With endotherms like humans, increased temperature generally may cause increased heart rate. Decreased temperature causes decreased heart rate.

Answer 2

increased temperature increases the heart rate as the viscosity of the blood decreases and consequentially the peripheral resistance decreases (poiseulle's equation). this effect is however negligible in endothermic organisms like humans as in the same the core temperature is maintained due to homeostasis except in pathological conditions like fever when the cardiac output increases as well.