Effects of selective antagonism of β‐adrenoceptor sub‐types on responses to isoprenaline in rat distal colon in vitro

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Abstract
1 The effects of the β1- and β2-adrenoceptor selective antagonists, CGP 20712A and ICI 118551 respectively, on responses to isoprenaline-induced relaxation of rat distal colon were investigated in order to determine the contributions of these subtypes to relaxation. In addition, the properties of ICI D7114, a novel putative stimulant of atypical β-adrenoceptors, were investigated. Our preliminary experiments with ICI D7114 showed that this compound lacked agonist activity in rat distal colon and in fact antagonized responses to isoprenaline. We therefore studied the antagonism of isoprenaline by ICI D7114 in more detail. 2 Longitudinal segments of rat distal colon were suspended in Krebs solution at 37°C for isometric recording. The Krebs solution contained EDTA (23 μm) and prazosin (0.1 μm) and was gassed with 95/5% O2/CO2. After an initial equilibration period, reproducible contractions to a submaximal concentration of methacholine (1 μm) were obtained before carrying out a concentration-response curve (CRC) to isoprenaline in a non-cumulative manner. Four consecutive CRCs to isoprenaline were carried out in each tissue with a 1 h interval between each curve. Antagonists were present in increasing concentrations during the intervals between CRCs. Control tissues received no antagonists to allow estimation of the magnitude of time-dependent changes. 3 Isoprenaline produced a concentration-dependent inhibition of methacholine-induced contractions. CRCs to isoprenaline were reproducible with no significant time-dependent changes. Propranolol produced no shift of the isoprenaline CRC at 0.01 μm and a 5 fold shift at 0.1 μm. No further shift was observed with 1 μm. CGP 20712A had no effect on the CRC to isoprenaline at 0.1, 1 and 3 μm. ICI 118551 produced little or no shift at 0.1 μm and a six fold shift with 1 μm. No further shift was observed with 3 μm. ICI D7114 produced a concentration-dependent parallel rightward shift of the CRC to isoprenaline. Schild analysis gave a slope close to unity and a mean pA2 value of 7.29 for ICI D7114. 4 The results with propranolol and β1- and β2-adrenoceptor antagonists confirm the mainly atypical nature of β-adrenoceptors in rat distal colon. There may also be a small contribution from β2-adrenoceptors in the response to isoprenaline but β1-adrenoceptors are absent. ICI D7114 has no agonist activity and behaves as a relatively high affinity reversible competitive antagonist of atypical β-adrenoceptors in this preparation.