An ionotropic effect can be applied to the effect of a transmitter substance or hormone on its target. The transmitter or hormone activates or deactivates ionotropic receptors (ligand-gated ion channels). The effect can be either positive or negative, specifically a depolarization or a hyperpolarization respectively. This term is commonly confused with an inotropic effect, which refers to a change in the force of contraction (e.g. in heart muscle ) produced by transmitter substances or hormones.
This term could be used to describe the action of acetylcholine on nicotinic receptors, glutamate on NMDA receptors or GABA on GABAa receptors.
Noradrenaline (a.k.a. norepinephrine) is an inotrope and has a positive inotropic effect on heart muscle, when binding to beta-1 adrenergic receptors on this tissue. The result is an increased cardiac output.
NOTE: As previously mentioned in this article, Inotrope is not the same as Ionotrope. However, with the similarity in spelling they are easily confused, as noted in the example above. Inotropes stimulate cardiac muscle (i.e. digoxin is an inotropic agent) whereas ionotropic agents generally requires a ligand to bind causing opening/closing of a ligand receptor gated channel.
- Neuroscience (Dale Purves), Third Edition, table 20:2