Article date: August 1993
By: Norman Lattimer, Paul Gupta, Keith F. Rhodes, in Volume 109, Issue 4, pages 1192-1195
5‐Hydroxytryptamine (5‐HT), in the presence of propranolol (1 μm), atropine (3 μm) and ketanserin (1 μm), induced a positive inotropic response of guinea‐pig isolated electrically paced left atria (pEC50 = 7.52). The positive inotropic response was mimicked by α‐methyl‐5‐HT (pEC50 = 7.26) and 5‐carboxamidotryptamine (5‐CT; pEC50 = 6.56) but not by sumatriptan or 1‐(m‐chlorophenyl) piperazine (m‐CPP).
The 5‐HT induced positive inotropic response was competitively antagonized by both mesulergine (pA2 = 7.68) and methiothepin (pA2 = 6.67). Methysergide was a surmountable antagonist at 3 nm producing a rightward shift in the 5‐HT concentration‐response curve giving an apparent pA2 = 9.2 with no significant reduction in the maximum. At higher concentrations, methysergide behaved as an insurmountable antagonist, significantly reducing the maximum response to 5‐HT as well as producing rightward shifts in the 5‐HT concentration‐response curves.
The 5‐HT‐induced positive inotropic response was not antagonized by either tropisetron (10 μm) or yohimbine (10 μm).
The guinea‐pig atrial 5‐HT receptor does not satisfy the criteria for any of the currently recognised 5‐HT receptor subtypes and appears to have some similarities to the atypical 5‐HT receptors previously described in other peripheral tissues.
5‐Hydroxytryptamine (5‐HT), in the presence of propranolol (1 μm), atropine (3 μm) and ketanserin (1 μm), induced a positive inotropic response of guinea‐pig isolated electrically paced left atria (pEC50 = 7.52). The positive inotropic response was mimicked by α‐methyl‐5‐HT (pEC50 = 7.26) and 5‐carboxamidotryptamine (5‐CT; pEC50 = 6.56) but not by sumatriptan or 1‐(m‐chlorophenyl) piperazine (m‐CPP).
The 5‐HT induced positive inotropic response was competitively antagonized by both mesulergine (pA2 = 7.68) and methiothepin (pA2 = 6.67). Methysergide was a surmountable antagonist at 3 nm producing a rightward shift in the 5‐HT concentration‐response curve giving an apparent pA2 = 9.2 with no significant reduction in the maximum. At higher concentrations, methysergide behaved as an insurmountable antagonist, significantly reducing the maximum response to 5‐HT as well as producing rightward shifts in the 5‐HT concentration‐response curves.
The 5‐HT‐induced positive inotropic response was not antagonized by either tropisetron (10 μm) or yohimbine (10 μm).
The guinea‐pig atrial 5‐HT receptor does not satisfy the criteria for any of the currently recognised 5‐HT receptor subtypes and appears to have some similarities to the atypical 5‐HT receptors previously described in other peripheral tissues.
DOI: 10.1111/j.1476-5381.1993.tb13748.x
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