ATP excites mouse vomeronasal sensory neurons through activation of P2X receptors.

Purinergic signaling through activation of P2X and P2Y receptors is critically important in the chemical senses. In the mouse main olfactory epithelium (MOE), adenosine 5'-triphosphate (ATP) elicits an increase in intracellular calcium ([Ca(2+)](I)) and reduces the responsiveness of olfactory sensory neurons to odorants through activation of P2X and P2Y receptors. We investigated the role of purinergic signaling in vomeronasal sensory neuron (VSN)s from the mouse vomeronasal organ (VNO), an olfactory organ distinct from the MOE that responds to many conspecific chemical cues. Using a combination of calcium imaging and patch-clamp electrophysiology with isolated VSNs, we demonstrated that ATP elicits an increase in [Ca(2+)](I) and an inward current with similar EC(50)s. Neither adenosine nor the P2Y receptor ligands adenosine 5'-diphosphate, uridine 5'-triphosphate, and uridine-5'-disphosphate could mimic either effect of ATP. Moreover, the increase in [Ca(2+)](I) required the presence of extracellular calcium and the inward current elicited by ATP was partially blocked by the P2X receptor antagonists pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate and 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate. Consistent with the activation of P2X receptors, we detected gene expression of the P2X1 and 3 receptors in the VNO by Reverse transcription polymerase chain reaction (RT-PCR). When co-delivered with dilute urine, a natural stimulus, ATP significantly increased the inward current above that elicited by dilute urine or ATP alone. Mechanical stimulation of the VNO induced the release of ATP, detected by luciferin-luciferase luminometry, and this release of ATP was completely abolished in the presence of the connexin/pannexin hemichannel blocker, carbenoxolone. We conclude that the release of ATP could occur during the activity of the vasomotor pump that facilitates the movement of chemicals into the VNO for detection by VSNs. This mechanism could lead to a global increase in excitability and the chemosensory response in VSNs through activation of P2X receptors.

Effect of topoisomerase modulators on cisplatin cytotoxicity in human ovarian carcinoma cells.

The in vitro interaction of modulators of topoisomerase I and II with cisplatin in human ovarian carcinoma cells might be synergistic. The interactions were evaluated by median effect analysis of survival data derived from continuous exposure to drug combinations for 10 days in colony-forming assays. The interaction between cisplatin and the topoisomerase I inhibitor camptothecin and the topoisomerase I activator beta-lapachone was additive, as was that between cisplatin and the topoisomerase II inhibitor novobiocin. Despite the clinical efficacy of the combination of etoposide (a topoisomerase II inhibitor) and cisplatin, the combination index at 50% cell kill indicated antagonism between these two drugs. Thus, biochemical synergism at the cellular level is not a prerequisite of improved therapeutic efficacy.

Dipyridamole enhancement of etoposide sensitivity.

Dipyridamole (DPM) enhanced the sensitivity of human ovarian carcinoma 2008 cells to etoposide (VP-16) producing a 5.5-fold reduction in 50% inhibitory concentration at a DPM concentration of 20 microM. This interaction was shown to be truly synergistic by isobologram and median effect analysis. DPM increased the steady-state VP-16 content of 2008 cells; a DPM concentration of 4 microM increased VP-16 content by 2-fold. DPM was 25 times less potent when cells were incubated in human plasma. In tissue culture medium 96% of the DPM was free, whereas in plasma only 15% was non-protein bound. DPM did not displace VP-16 from proteins under either condition. DPM did not increase the initial influx of VP-16 but did inhibit the initial efflux, reducing the efflux rate constant by 27%. DPM had no effect on the later stages of drug efflux, nor did it irreversibly bind VP-16 in the cell. The effect of DPM was evident within 1 min; once removed, the effect disappeared within 2 min. DPM is a potent nucleoside membrane transport inhibitor and can also inhibit cyclic AMP (cAMP) phosphodiesterase in platelets. Nitrobenzylthioinosine, another nucleoside transport inhibitor which competes for binding with DPM, did not enhance sensitivity to VP-16 or increase VP-16 cellular accumulation and did not block the effect of DPM. In 2008 cells, DPM did not increase cAMP; when cAMP was increased by incubation with dibutyryl cyclic 3':5'-AMP, there was no synergy with VP-16. The results indicate that enhanced sensitivity to VP-16 was not due to an effect of DPM on the protein binding of VP-16 or on cellular cAMP and suggest that it is not directly related to inhibition of nucleoside transport. This effect appears to be a newly identified mechanism of action for this agent.

Comparison of the synergistic potentiation of etoposide, doxorubicin, and vinblastine cytotoxicity by dipyridamole.

Dipyridamole (DPM) enhanced sensitivity to etoposide (VP-16), doxorubicin (DOX), and vinblastine (VBL) in a human ovarian carcinoma cell line that was already relatively sensitive to all three agents. This interaction was shown to be truly synergistic by median effect analysis over a 2 log cell kill. The combination index at 50% cell kill (CI50) was used to quantitate the extent of synergy. The CI50s were 0.42, 0.66, and 0.30 for VP-16, DOX, and VBL, respectively. We compared the effect of DPM on the cellular pharmacology of each chemotherapeutic drug. DPM increased the steady state cellular content of VP-16 by a maximum of 3.2-fold, and that of DOX and VBL by 1.7- and 3.7-fold, respectively. There was a good correlation between the CI50 and the DPM-induced increase in cellular drug content (r = 0.94). DPM had no effect on the initial influx VP-16 or DOX but did increase the initial influx of VBL by 3.5-fold. DPM inhibited the initial efflux of all three compounds. However, there was no relation between the extent of efflux inhibition and the magnitude of the DPM-induced increase in cellular drug content, indicating that DPM must have other effects as well. DPM has chemical characteristics similar to other known modulators of VP-16, DOX, and VBL sensitivity. When compared to verapamil, DPM was as efficacious but twice as potent in its synergistic enhancement of VP-16 sensitivity. These results demonstrate that DPM can markedly increase the cytotoxicity of VP-16, DOX, and VBL and suggest possible clinical applications.