Red blood cells of the lizards Ameiva ameiva (Squamata, Teiidae) display multiple mechanisms to control cytosolic calcium

We have previously reported that lizard red blood cells control their cytosolic calcium concentration by sequestering calcium ions in pools, which could be discharged by thapsigargin, by the Na+/H+ ionophore, monensin, by the K+/H+ ionophore, nigericin and by the proton pump inhibitor, bafilomycin A1 [1]. We have now demonstrated, with the aid of confocal microscopy, the presence in these cells of organelles, which accumulate the dye acridine orange and are thus by inference the sites of proton pools. We have found, moreover, that monensin, nigericin and bafilomycin all act to discharge these pools. We further show that calcium release ensues when the calcium ionophore, ionomycin, is added after thapsigargin and monensin; this implies the existence of a third pool, besides the acidic pool and the Endoplasmic Reticulum (ER), which participates in calcium homeostasis. The ER calcium pool can de discharged by the addition of the second messenger, IP3, and we present evidence, based on confocal microscopy, that the IP3 receptors are located in or close to the nucleus.

Signal transduction in red bloodcells of the lizards Ameiva ameiva and Tupinambis merianae (Squamata, Teiidae)

The fluorescent calcium probe, Fluo-3, AM was used to measure the intracellular calcium concentration in red blood cells (RBCs) of the teiid lizards Ameiva ameiva and Tupinambis merianae. The cytosolic [Ca2+] is maintained around 20 nM and the cells contain membrane-bound Ca2+ pools. One pool appears to be identifiable with the endoplasmic reticulum (ER) inasmuch as addition of the sarco-endoplasmic reticulum Ca2+ ATPase, SERCA, inhibitor thapsigargin induces an increase in cytosolic [Ca2+ both in the presence and in the absence of extracellular Ca2+. In addition to the ER, an acidic compartment appears to be involved in Ca2+ storage, as collapse of intracellular pHgradients by monensin, a Na+ -H+ exchanger, and nigericin, a K+ -H+ exchanger, induce the release of Ca2+ from internal pools. A vacuolar H+ pump, sensitive to NBD-Cl and bafilomycin appears to be necessary to load the acidic Ca2+ pools. Finally, the purinergic agonist ATP triggers a rapid and transient increase of [Ca2+]c in the cells from both lizard species, mostly by mobilization of the cation from internal stores.