Schematic representation of mechanism of action of inotropes on cardiac myocytes. Stimulation/inhibition of cell receptors alters enzyme (in black boxes) activity which in turn alters the availability of substrates to facilitate calcium entry into the sarcoplasmic reticulum and subsequent force of myocardial contraction. Agents affecting receptor or pump activity are shown in dotted boxes.α₁, alpha 1 adrenoreceptor; α₂, alpha 2 adrenoreceptor; Gx, various G proteins (s, i, q); MR, various myocyte receptors (including beta adrenoreceptors 5HT receptors, VIP, somatostatin, M2 muscurinic); GR, glucagon receptor; Na/K pump, sodium potassium exchange pump; θ, inhibits; Ca²⁺, calcium; PDE, phosphodiesterase; SR, sarcoplasmic reticulum.