Which of the following physiological processes is a direct example of cell-to-cell signalling in homeostatic regulation?
Binding of adrenaline to specific beta-adrenergic receptors on cardiac muscle cells, initiating an intracellular cascade
Facilitated diffusion of potassium ions (K+\text{K}^+K+) out of the axon through voltage-gated channels during repolarisation
Passive diffusion of carbon dioxide from respiring tissues into blood capillaries down a partial pressure gradient
Active transport of calcium ions (Ca2+\text{Ca}^{2+}Ca2+) from the sarcoplasm into the sarcoplasmic reticulum of a muscle fibre
