Chemical regulation of the heart in animals
Cations
Na+ and K+ ions favour diastole, whereas Ca++ ion favours systole. Sudden infusion of the Ca++ ions into the heart causes calcium rigor or systolic arrest.
Catecholamines
Epinephrine and norepinephrine are the neurotransmitters of sympathetic division, acts on β1 receptors and exerts positive chronotropic (atria and ventricles) and inotropic effects on heart. β1 receptors are found in S.A. node, A. V. node and the ventricles.
Stimulation of α1 receptor(SA node, myocardium) causes positive inotropic effect in the atrial and ventricular muscles. Catecholamine increases the stroke volume and cardiac output during emergency / stress.
Acetyl choline
The neurotransmitter of parasympathetic division exhibits negative chronotropic (decreased rate) and inotropic (decreased force of atrial contraction) effects, thus decreases cardiac output.
Metabolic acid end products
CO2 and other metabolic acid products, H+ ions, lactic acid lower the pH below normal, causes increased rate and force of contraction of the heart.