MOBILIZATION OF CA2+ INFLUX, BUT NOT OF STORED CA2+, BY EXTRACELLULAR ATP IN RAT SUBMANDIBULAR-GLAND ACINI

To determine its effect on cytosolic Ca2+ levels (Ca-i(2+)) and to assess its role as an autonomic cotransmitter, ATP was added to suspensions of rat pancreatic or submandibular gland acini loaded with fura-2. ATP had no effect on pancreatic acinar Ca-i(2+) at either high (2.5 mM) or low (<10 nM) extracellular Ca2+ concentrations (Ca-o(2+)). In submandibular acini, ATP had little effect on Ca-i(2+) when Ca-o(2+) was 10 nM but, with Ca-o(2+) equal to 2.5 mM, ATP increased Ca-i(2+) to a steady-state level 2-4 times the resting level of 50-100 nM. Addition of carbachol or epinephrine further increased Ca-i(2+). The mean and standard deviation of the ATP(4-) concentration at half of maximal activity (K-0.5ATP4-) was 33 +/- 7 mu M and the Hill coefficient was 1.5 +/- 0.7 (n = 9). The active species is apparently ATP(4-), as adding Mg2+ increases the total concentration of ATP needed to elevate Ca-i(2+). GTP, UTP, ADP and adenosine produced no significant changes in Ca-i(2+). However, 3'-O-(4-benzoyl)benzoyl ATP (BZATP) increased Ca-i(2+) with a K-0.5BZATP of 2.4 +/- 0.5 mu M and a Hill coefficient of 2.8 +/- 0.4 (n = 8). ATP-induced changes in Ca-i(2+) were not due to a generalized increase in acinar-cell permeability and were unaffected by voltage-dependent Ca2+ channel blockers or by the presence of depolarizing concentrations of Kf. It is concluded that ATP(4-) increases Catt in rat submandibular gland acini by interaction with a purinoceptor of the P2Z subtype, that ATP(4-) has little effect on stores of Ca2+ within the acinar cells but promotes Ca2+ influx to increase Ca-i(2+), an increase that is augmented by the simultaneous presence of carbachol or epinephrine.