Adenosine A1 receptor mediated formation of inosito 1,4,5-trisphosphate (Ins(1,4,5)P3) and accumulation of cytoplasmic Ca2+ ([Ca2+]i) were investigated in DDT1 MF-2 smooth muscle cells. A strong reduction of the adenosine and N6-cyclopentyladenosine (CPA) induced rise in [Ca2+]i was observed after blocking Ca2+ entry across the plasma membrane with LaCl3. This effect of LaCl3 was not observed in the absence of extracellular Ca2+; it was not caused by reduced Ins(1,4,5)P3 formation or changed Ins(1,4,5)P3 induced Ca2+ release, or influenced by temperature. The inhibition of the CPA induced increase in [Ca2+]i by LaCl3 was strongly counteracted in the presence of ortho-vanadate, an inhibitor of plasma membrane Ca2+ ATPase. Ortho-vanadate might also reduce protein tyrosine-phosphate phosphatase activity involved in tyrosine kinase mediated phospholipase C (PLC) activation. However, ortho-vanadate and tyrphostin 25, a tyrosine kinase inhibitor, did not affect the CPA induced formation of Ins(1,4,5)P3. Taken together, these results show a strong contribution of Ca2+ pumping across the plasma membrane to the regulation of [Ca2+]i mediated by adenosine A1 receptors. Na+/Ca2+ exchange only played a minor role in the initial phase of CPA induced Ca2+ metabolism as measured in low Na+ containing solution. The mechanism by which adenosine A1 receptors activate plasma membrane Ca2+ ATPase pumps does not include direct stimulation of pumps, but most likely involves an indirect pathway activated by a rapid increase in [Ca2+]i.