Srinivas M, Shryock J C, Dennis D M, Baker S P, Belardinelli L
Department of Pharmacology, University of Florida, Gainesville, Florida 32610, USA.
Mol Pharmacol. 1997 Oct;52(4):683-91. doi: 10.1124/mol.52.4.683.
Adenosine activates adenosine-induced inwardly rectifying K+ current (IKAdo) and inhibits isoproterenol (100 nM)-stimulated L-type Ca2+ current (beta-ICa,L) of guinea pig atrial myocytes with EC50 values of 2.17 and 0.20 microM, respectively. We determined whether this 11-fold difference in potency of adenosine is due to the existence of a greater A1 adenosine receptor reserve for the inhibition of beta-ICa,L than for the activation of IKAdo. Atrial myocytes were pretreated with vehicle (control) or the irreversible A1 adenosine receptor antagonist 8-cyclopentyl-3-[3-[[4-(fluorosulfonyl)benzoyl]oxy]propyl]-1-propylxa nthine (FSCPX) (10 and 50 nM) for 30 min, and after a 60-min washout period, concentration-response curves were determined for the adenosine-induced activation of IKAdo and inhibition of beta-ICa,L. Pretreatment of atrial myocytes with 10 nM FSCPX reduced the maximal activation of IKAdo by 60% (7.9 +/- 0.2 to 3.2 +/- 0.1 pA/pF). In contrast, a higher concentration of FSCPX (50 nM) was required to reduce the maximal inhibition of beta-ICa,L by 39% (95 +/- 4% to 58. 7 +/- 5.6%) and caused a 15-fold increase in the EC50 value of adenosine. Values of the equilibrium dissociation constant (KA) for adenosine to activate IKAdo and inhibit beta-ICa,L, estimated according to the method of Furchgott, were 2.7 and 5.6 microM, respectively. These values were used to determine the relationship between adenosine receptor occupancy and response. Half-maximal and maximal activations of IKAdo required occupancies of 40% and 98% of A1 adenosine receptors, respectively. In contrast, occupancies of only 4% and 70%, respectively, of A1 adenosine receptors were sufficient to cause half-maximal and maximal inhibitions of beta-ICa, L. Consistent with this result, a partial agonist of the A1 adenosine receptor SHA040 inhibited beta-ICa,L by 60 +/- 3.5% but activated IKAdo by only 18.1 +/- 2.5%. The results indicate that the A1 adenosine receptor is coupled more efficiently to an inhibition of beta-ICa,L than to an activation of IKAdo.
腺苷可激活豚鼠心房肌细胞的腺苷诱导内向整流钾电流（IKAdo），并抑制异丙肾上腺素（100 nM）刺激的L型钙电流（β-ICa,L），其半数有效浓度（EC50）值分别为2.17和0.20 μM。我们研究了腺苷在效力上11倍的差异是否是由于存在更大的A1腺苷受体储备，用于抑制β-ICa,L而非激活IKAdo。心房肌细胞用溶剂（对照）或不可逆的A1腺苷受体拮抗剂8-环戊基-3-[3-[[4-(氟磺酰基)苯甲酰基]氧基]丙基]-1-丙基黄嘌呤（FSCPX）（10和50 nM）预处理30分钟，在60分钟洗脱期后，测定腺苷诱导的IKAdo激活和β-ICa,L抑制的浓度-反应曲线。用10 nM FSCPX预处理心房肌细胞可使IKAdo的最大激活降低60%（从7.9±0.2 pA/pF降至3.2±0.1 pA/pF）。相比之下，需要更高浓度的FSCPX（50 nM）才能使β-ICa,L的最大抑制降低39%（从95±4%降至58.7±5.6%），并使腺苷的EC50值增加15倍。根据Furchgott方法估算的腺苷激活IKAdo和抑制β-ICa,L的平衡解离常数（KA）值分别为2.7和5.6 μM。这些值用于确定腺苷受体占有率与反应之间的关系。IKAdo的半数最大激活和最大激活分别需要占据40%和98%的A1腺苷受体。相比之下，仅占据4%和70%的A1腺苷受体就足以分别导致β-ICa,L的半数最大抑制和最大抑制。与该结果一致，A1腺苷受体的部分激动剂SHA040可使β-ICa,L抑制60±3.5%，但仅使IKAdo激活18.1±2.5%。结果表明，A1腺苷受体与β-ICa,L抑制的偶联比与IKAdo激活的偶联更有效。
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