Final answer:
PKA increases relaxation rates in myocytes by triggering processes that provide a quick energy source, and by influencing calcium handling, allowing for faster relaxation post contraction.
Step-by-step explanation:
Protein Kinase A (PKA) plays a significant role in increasing relaxation rates in myocytes, specifically cardiac muscle cells. The influence of PKA on relaxation rates is closely tied to its action within multiple cellular signaling pathways. When adrenaline binds to ß-adrenergic receptors, it leads to a cascade that elevates cyclic AMP (cAMP) levels, which in turn activates PKA. PKA phosphorylates various target proteins, leading to different cellular responses.
One of the crucial pathways involves the degradation of glycogen, providing a quick source of glucose for energy. PKA activates glycogen phosphorylase kinase (GPK), which in turn activates glycogen phosphorylase (GP), leading to the breakdown of glycogen into glucose. Simultaneously, PKA inhibits glycogen synthase (GS), preventing the futile cycle of glycogen synthesis from glucose. This increased availability of glucose ensures that the muscle cell has an immediate energy source to respond to the fight-or-flight reflex initiated by adrenaline.