Pharmacological characterization and autoradiographic localization of a putative dopamine D4 receptor in the heart

1 The pharmacological profile and the anatomical localization of a putative dopamine D4 receptor were assessed in sections of rat and human atria and ventricles using combined radioligand binding and autoradiographic techniques with [3H]-spiperone as a ligand. 2 [3H]-Spiperone was bound specifically to sections of human and rat atria and ventricles. The binding was time-, temperature- and concentration-dependent, belonging to a single class of high-affinity sites. In atria, the dissociation constant value (K(d)) was 0.45 nM in rats and 0.32 nM in humans, and the maximum density of binding sites (B(max)) was 31.6 ± 2.9 fmol mg-4 tissue in rats and 18.8 ± 0.7 fmol mg-1 tissue in humans. In ventricles, K(d) was 0.38 nM in rats and 0.39 nM in humans, and the B(max) was 43.5 ± 3.0 fmol mg-1 tissue in rats and 56.4 ± 3.2 fmol mg-1 tissue in humans. 3 The pharmacological profile of [3H]-spiperone binding to sections of both rat and human atria and ventricles was consistent with the labelling of dopamine D2-like receptors. [3H]-Spiperone binding was more sensitive to displacement by the neuroleptic clozapine in sections of atria than of ventricles, suggesting the expression of a dopamine D4 receptor in atrial tissue. Moreover, preincubation of some sections with a dopamine D4 receptor antibody and subsequent exposure to [3H]-spiperone caused a remarkable decrease of radioligand binding to sections of atria, but only a slight reduction of binding to sections of ventricles. 4 Light microscope autoradiography revealed the accumulation of silver grains over atrial tissue within atrial myocardiocytes. A higher density of silver grains was developed in rat than in human atria. In ventricles, silver grains were accumulated primarily in cluster areas both in rats and in humans. 5 The above findings suggest the expression of a dopamine D4 receptor in rat atria, but not in ventricles. A similar site was identified in human atria. The possible relevance of a dopamine D4 receptor in the heart is discussed.