The purpose of this study was to determine the effects of local anesthetics (lidocaine and procaine) and hypothermic perfusion on Na+/Ca2+ exchanger activity during reperfusion-induced calcium paradox in vitro. Hypothetically, this work provides a mechanism for reperfusion paradox injury in vitro that may relate to a variety of ischemic insults in vivo. Intracellular calcium ([Ca2+]i) in individual cells was measured using a fura-2 photometry system during protocols previously shown to dramatically and persistently increase [Ca2+]i in a manner that replicates certain phases of reperfusion injury in vivo. Our results suggest that short (< 5.0 min) exposure to hypocalcia prompts subsequent, potentially lethal, sodium-dependent calcium entry via the reverse phase of the Na+/Ca2+ exchanger. The local anesthetics lidocaine or procaine are able to block [Ca2+]i increases when added at the time of reperfusion. Furthermore, since [Ca2+]i is normally increased when local anesthetics are added by themselves, the results suggest that local anesthetics promote the forward mode of Na+/Ca2+ exchange (i.e., Na-dependent calcium removal) in addition to reverse Na+/Ca2+ exchanger blockade. Low temperature (18 degrees C) perfusion or reperfusion also affects exchanger dynamics and attenuates calcium entry during such protocols. These experiments further implicate the Na+/Ca2+ exchanger in reperfusion-mediated cell injury and provide potential ways to therapeutically modify such phenomena in vivo.