Article date: July 1972
By: ANNA B. DRAKONTIDES, MICHAEL D. GERSHON in Volume 45, Issue 3, pages 417-434
The action and interaction of noradrenaline (NA), 5‐hydroxytryptamine (5‐HT) and the sympathetic innervation was studied in the isolated taenia of the guinea‐pig caecum.
Addition of 5‐HT led to a contraction of the taenia while addition of NA or perivascular nerve stimulation resulted in relaxation. Responses to 5‐HT or perivascular nerve stimulation were abolished by tetrodotoxin. Tetrodotoxin did not affect responses to applied NA. Hexamethonium and hyoscine converted the 5‐HT response to a relaxation and augmented the relaxation which followed low frequency perivascular nerve stimulation. Hexamethonium and hyoscine did not affect the dose‐response relationship for NA.
Fatigue of mechanical responses of the taenia to perivascular nerve stimulation was accelerated when nerves were stimulated in the presence of 5‐HT or α‐methyl‐p‐tyrosine (α‐MPT). These two agents were additive in this action.
Reserpine, 6‐hydroxydopamine and α‐MPT all reduced the NA content of the taenia. However, only after 6‐hydroxydopamine could adrenergic activity be related to NA content.
Segments of taenia were incubated with either tritiated NA or 5‐HT. An increased rate of release of radioactivity followed perivascular nerve stimulation after incubation with either substance. This release did not occur when tissue was taken from animals given reserpine or 6‐hydroxydopamine.
It is concluded that 5‐HT activates neural elements exclusively while NA has a direct effect on smooth muscle. 5‐HT can apparently be taken up by adrenergic axons, and appears to enter the releasable neurotransmitter pool. Since none of the actions characteristic of 5‐HT are seen when it is released by adrenergic axons as a false neurotransmitter, the released amine probably fails to reach neuronal receptors for 5‐HT.
The action and interaction of noradrenaline (NA), 5‐hydroxytryptamine (5‐HT) and the sympathetic innervation was studied in the isolated taenia of the guinea‐pig caecum.
Addition of 5‐HT led to a contraction of the taenia while addition of NA or perivascular nerve stimulation resulted in relaxation. Responses to 5‐HT or perivascular nerve stimulation were abolished by tetrodotoxin. Tetrodotoxin did not affect responses to applied NA. Hexamethonium and hyoscine converted the 5‐HT response to a relaxation and augmented the relaxation which followed low frequency perivascular nerve stimulation. Hexamethonium and hyoscine did not affect the dose‐response relationship for NA.
Fatigue of mechanical responses of the taenia to perivascular nerve stimulation was accelerated when nerves were stimulated in the presence of 5‐HT or α‐methyl‐p‐tyrosine (α‐MPT). These two agents were additive in this action.
Reserpine, 6‐hydroxydopamine and α‐MPT all reduced the NA content of the taenia. However, only after 6‐hydroxydopamine could adrenergic activity be related to NA content.
Segments of taenia were incubated with either tritiated NA or 5‐HT. An increased rate of release of radioactivity followed perivascular nerve stimulation after incubation with either substance. This release did not occur when tissue was taken from animals given reserpine or 6‐hydroxydopamine.
It is concluded that 5‐HT activates neural elements exclusively while NA has a direct effect on smooth muscle. 5‐HT can apparently be taken up by adrenergic axons, and appears to enter the releasable neurotransmitter pool. Since none of the actions characteristic of 5‐HT are seen when it is released by adrenergic axons as a false neurotransmitter, the released amine probably fails to reach neuronal receptors for 5‐HT.
DOI: 10.1111/j.1476-5381.1972.tb08098.x
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