Contribution of cellular and extracellular Ca2+ during 5-hydroxytryptamine-induced contractions of rabbit ear artery

The ability of 5-hydroxytryptamine (5-HT) as compared to norepinephrine (NE) to mobilize different Ca2+ pools was investigated in rabbit ear artery (REA). 5-HT appears to both mobilize intracellular Ca2+ and trigger transmembrane Ca2+ influx in this preparation. Although stimulation of alpha-adrenoceptors by 5-HT appears to be involved, the duration of 5-HT-induced cellular and extracellular Ca2+-dependent contractions outlasts the duration of those produced by equipotent concentrations of NE. Both cellular and extracellular Ca2+-dependent contractile responses produced by low but not high concentrations of 5-HT were significantly higher in arterial segments pretreated with 6-OH dopamine and in presence of desoxycorticosterone and propranolol while those produced by NE were unaffected. The duration of extracellular Ca2+-dependent 5-HT and NE-induced contractions was enhanced over the whole concentration range and in a similar manner. These findings as well as the results obtained with the receptor antagonists cyproheptadine and phentolamine could be interpreted as an indication that: (a) stimulation of alpha-adrenoceptors by 5-HT is only in part responsible for the contractile effects of 5-HT in REA; (b) additional mechanism(s) activated by 5-HT appear to modulate the duration of both cellular and extracellular Ca2+-dependent contractions and, (c) difference(s) in tissular handling of these amines do not appear to play a role in their different contractile effects.