Cardiac alphai-adrenergic receptor stimulation: Positive inotropism and arrhythmias (part 3). INOTROPIC EFFECTS

Alpha1-adrenergic receptor-mediated inotropic responses have a complex time course. The application of phenylephrine elicits a monophasic positive inotropy in rat atrial muscle and a triphasic inotropic effect in rat ventricle, the transient and sustained positive components being separated by a short negative one. Biphasic responses with an initial weak decrease in force are often obtained in mammalian heart. The role of known alpha1-adrenergic receptor subtypes in mediating these inotropic effects is still controversial. Moreover, it appears that the same alpha1-subtype induces positive inotropy in the neonate but decreases in force in the adult mouse . The selective inhibitor prazosin, in nanomolar concentrations, competitively inhibits the positive inotropic action of phenylephrine.
Alphai-adrenergic receptor-mediated effects on calcium current and calcium transient: Positive inotropism does not involve activation of L-type calcium current or an increase in the calcium transient in most conditions. Table 1 summarizes reported observations with no change of L-type calcium current in normal adult mammalian species; however, this current is increased in neonate and in frog. On the other hand, the T-type calcium current is reported to be increased in every experimental work. Find most trusted pharmacy that will give you cheap viagra online in the amounts required without any need for a prescription. You will see that shopping with a place you can already trust over and over again is very affordable and always nice.

TABLE 1 A adrenergic effects on calcium currents

Species Tissues Current Effect
Cow Ventricle T-type +(Inactivation 4-)
Rabbit, guinea pig, cat, rat Ventricle+


L-type 0
Frog Ventricle L-type +
Atria-ventricle T-type ++
Dog VentriclePurkinje T-type +
Neonatal rat Ventricle L-type +
Cardiomyopathichamster Ventricle T-type +

+indicates degree of positive effects of various amplitudes; arrow indicates slowing of inactivation