Pharmacological characterization of the effects of taurine on calcium uptake in the rat retina.
Taurine is known to increase ATP-dependent calcium ion (Ca2+) uptake in retinal membrane preparations and in isolated rod outer segments (ROS) under low calcium conditions (10 microM) (Pasantes-Morales and Ordóñez, 1982; Lombardini, 1991). In this report, ATP-dependent Ca2+ uptake in retinal membrane preparations was found to be inhibited by 5 microM cadmium (Cd2+), suggesting the involvement of cation channel activation. The activation of cGMP-gated cation channels, which are found in the ROS, is a crucial step in the phototransduction process. An inhibitor of cGMP-gated channels, LY83583, was found to inhibit taurine-stimulated ATP-dependent Ca2+ uptake but had no effect on ATP-dependent Ca2+ uptake in the absence of taurine, indicating that taurine may be increasing ATP-dependent Ca2+ uptake through a mechanism of action involving the opening of cGMP-gated channels. The activation of cGMP-gated channels with dibutyryl-cGMP and with phosphodiesterase inhibition using zaprinast caused an increase in ATP-dependent Ca2+ uptake in isolated ROS, but not in taurine-stimulated ATP-dependent Ca2+ uptake. LY83583 had the same effects in isolated ROS as in retinal membrane preparations. Another inhibitor of cGMP-gated channels, Rp-8-Br-PET-cGMPS, produced the same pattern of inhibition in isolated ROS as LY83583. Thus, there appears to be a causal link between taurine and the activation of the cGMP-gated channels in the ROS under conditions of low calcium concentration, a connection that suggests an important role for taurine in the visual signalling function of the retina.
Militante
J D
JD
Department of Pharmacology, Texas Tech University Health Sciences Center, Lubbock, USA.
Lombardini
J B
JB
D000255:Adenosine Triphosphate / Q000378:metabolism; D000634:Aminoquinolines / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects*; D006152:Cyclic GMP / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D015640:Ion Channel Gating; D051381:Rats; D017208:Rats, Wistar; D012374:Rod Cell Outer Segment / Q000378:metabolism*; D013654:Taurine / Q000494:pharmacology*
D000634:Aminoquinolines; D015220:Calcium Channels; D002104:Cadmium; D013654:Taurine; D000255:Adenosine Triphosphate; C041715:6-anilino-5,8-quinolinedione; D006152:Cyclic GMP; D002118:Calcium