Analysis of the atypical characteristics of adenosine receptors mediating negative inotropic and chronotropic responses of guinea-pig isolated atria and papillary muscles

Abstract

1. Adenosine receptor(s) mediating negative inotropy of paced left atria, isoprenaline-stimulated paced left atria and papillary muscles, and negative chronotropy of spontaneously beating right atria were characterized. 2. Isometric tension of guinea-pig isolated paced left atria and left ventricular papillary muscles and rate of contraction of spontaneously beating right atria were recorded. Papillary muscles were pre-stimulated with isoprenaline (1x10-8 M). Concentration-response curves (CRCs) for tension or rate reduction by N6-cyclopentyladenosine (CPA), the stereoisomers of N6-(2-phenylisopropyl)adenosine ((+)-PIA and (-)-PIA), 5'-(N-carboxamido)adenosine (NECA), N6-2-(4-aminophenyl)ethyladenosine (APNEA) and N6-(3-iodobenzyl)adenosine-5'-N-methyuromide (IB-MECA) revealed a potency order of CPA=(-)-PIA>NECA in right atria and papillary muscles, which is consistent with involvement of A1-receptors. The potency order in left atria was CPA=NECA>(-)-PIA>(+)-PIA>APNEA, which is not typical of A1 adenosine receptors. Weak activity of APNEA and IB-MECA discounts involvement of A3 receptors. 3. pA2 values for the antagonism of CPA by 8(p-sulfophenyl)theophylline (8-SPT) were calculated from Schild plots (log concentration-ratio against log 8-SPT concentration), the unity slopes of which indicated competitive antagonism. The pA2 value in the papillary muscles was significantly higher than for atrial preparations, indicating a possible difference in receptor characteristics between atrial and papillary muscle responses. 4. In left and right atria there was a limit to the displacement of the CPA CRCs at higher concentrations of 8-SPT. The 8-SPT-resistant component of the response is suggested to arise from duality of coupling of a common A1 receptor through either different G proteins or G protein subunits to independent transduction pathways. The results with papillary muscles can be explained by a typical A1 receptor coupled to a single transduction pathway.

Figure 1

Mean concentration-response curves (n⩾4) for the inhibition of (A) left atrial tension, (B) right atrial rate, (C) ISO-stimulated increase in papillary muscle tension by CPA (▪), (−)-PIA (▾), NECA (⧫), (+)-PIA (▴), APNEA (•) and IB-MECA (X). Each point is the mean reduction of tension or rate. Vertical bars represent the s.e.mean, their absence indicating they lie within the symbol.

Figure 2

Mean concentration-response curves (n⩾4) for the inhibition of (A) left atrial tension, (B) right atrial rate, (C) ISO-stimulated left atrial tension, (D) ISO-stimulated increase in papillary muscle tension by CPA in the absence (▪) and presence of 8-SPT; 10⁻⁶ (•), 3×10⁻⁶ M (X), 10⁻⁵ M (▴), 10⁻⁴ M (▾), 3×10⁻⁴ M (⧫), 10⁻³ M (+). Each point is the mean reduction of tension or rate. Vertical bars represent the s.e.mean, their absence indicating they lie within the symbol.

Figure 3

Schild plots for the antagonism of CPA by 8-SPT in (A) left atria, (B) right atria, (C) ISO-stimulated left atria, (D) ISO-stimulated papillary muscle. Mean (n>4) concentration-ratio (CR) for the shift of individual concentration-response curves in the presence of 8-SPT relative to the mean curve in its absence are plotted as log (CR-1) against log concentration (M) of 8-SPT. Vertical bars represent s.e.mean, their absence indicating that they lie within the symbol. Linear regression for all points are indicated by solid lines. Where the higher concentrations appear to deviate from this line, further regression lines are shown (dashed lines).