The A1 adenosine receptor (A1AR) exerts important biological effects in the mammalian biology. To provide insights into the role A1AR action in human physiology, we characterized the pharmacologic properties of the human A1AR, examined somatic sites of A1AR gene expression, and identified the chromosomal location of the human A1AR gene. Using stably transfected CHO cells, the ligand binding properties of human and rat A1ARs were directly compared. Saturation studies showed that the human and rat A1ARs had similar high affinity for the A1 agonist [(3)H]CCPA (human, K(d)=517±64 pM; B(max) 438±29 fmol/mg of protein; rat, K(d)=429±69 pM; B(max) 358±76 fmol/mg of protein). Competition studies performed using seven adenosine agonists and four adenosine antagonists also did not detect differences in the ligand binding properties among the rat and human A1ARs. Northern analysis of 16 human tissues revealed the presence of a single hybridizing transcript of 2.5 kb. Human A1AR receptor mRNA expression was greatest in brain and testis; lower levels of A1AR mRNA were present in heart, pancreas, kidney and spleen. Southern blotting and PCR analysis of human-rodent somatic cell hybrids showed that the A1AR gene is on human chromosome 1. Using fluorescence in situ hybridization, the human A1AR gene was further localized to the 1q32.1 region. These observations show that the human A1AR is a high affinity receptor that has ligand binding properties similar to the rat A1AR, human A1AR mRNA is heavily expressed in brain and testis, and the gene encoding the human A1AR is present on the long arm of chromosome 1.