Local Extracellular pH Research Articles

Changes in medullary extracellular pH, sympathetic and phrenic nerve activity during brainstem perfusion with CO 2 enriched solutions

Measurements are presented of sympathetic nerve activity (SNA), phrenic nerve activity (PNA), and local extracellular pH (ECF pH) within the rostral ventrolateral medulla (RVLM) in response to perfusions of the RVLM with CO 2-enriched saline. Experiments were performed on cats anaesthetized with chloralose. The ventrolateral medullary surface was exposed, and a catheter was placed in the left vertebral artery from the axilla to allow perfusion of the RVLM. Baroreceptor and peripheral chemoreceptor denervations were performed by cutting the vagal, aortic and carotid sinus nerves. The activities of the renal and the phrenic nerve were recorded, in some experiments in parallel with the cardiac nerve. Recordings of the pH were done with ion-sensitive theta-microelectrodes. A linear relationship between the CO 2 concentration of the perfusate and the evoked changes in ECF pH was found. The ECF pH did not change systematically in one or the other direction within depths between 1 and 3 mm below the surface of the medulla. The various patterns of interaction of ECF pH, SNA, and PNA are described in detail. Phrenic nerve response to perfusions was very variable; a more prolonged increase in amplitude of phasic discharges compared to the duration of changes in SNA and ECF pH was the most frequent finding, but non-phasic tonic activation and complete silence were also seen during perfusions. SNA could also deviate from ECF pH both with regard to its latency and to its time course in response to perfusions. Therefore, this study provides further evidence for deviations of cardiorespiratory adaptation from ECF pH, corroborating the notion that this parameter is not the decisive one for central chemoreception.

Different effects of respiratory and metabolic acidosis on preganglionic sympathetic nerve activity

We studied sympathetic nerve activity (SNA) responses, recorded in multifiber preparations of left third thoracic white ramus, to respiratory or isocapnic metabolic acidosis or to CO2 enhancement at constant pH in chloralose-anesthetized paralyzed artificially ventilated cats. Cardiopulmonary, baro-, and peripheral chemoreceptors were denervated by bilaterally cutting vagus and carotid sinus nerves. Acidosis was induced by either decreasing artificial ventilation or infusing HCl (0.5 M i.v.). Both respiratory and isocapnic metabolic acidosis induced a decrease in local extracellular pH, measured directly with pH-sensitive microelectrodes within medulla region containing sympathoexcitatory bulbospinal neurons. The magnitude of changes in medullary pH was independent of the way systemic acidosis was generated. Despite uniformity of changes in local medullary extracellular pH due to systemic respiratory or isocapnic metabolic acidosis, different responses were observed in preganglionic SNA. Isocapnic metabolic acidosis resulted in a slight increase in SNA, averaging 6.4% per 0.05 systemic pH unit decrease. In contrast, respiratory acidosis induced by decreasing artificial ventilation produced a more pronounced increase of SNA, reaching peak changes of approximately 70% compared with control level with normal blood gases, an average increase of 13% per 0.05 systemic pH unit decrease. We conclude that systemic CO2 and H+ concentrations represent different stimuli to sympathetic nervous system. Despite similar changes of local extracellular pH within rostral ventrolateral medulla during systemic acidosis, different responses of SNA suggest other sites or as yet unknown additional effects of CO2 as being responsible for excitation of sympathetic activity.

Changes in intra- or extracellular pH do not mediate P-glycoprotein-dependent multidrug resistance.

P-glycoprotein (Pgp)-mediated multidrug resistance (MDR) is thought to result from active extrusion of lipid-soluble, titratable chemotherapeutic agents. Given the lack of demonstration of coupling between ATP hydrolysis and drug transport, the resistance to chemically unrelated compounds, and findings of elevated intracellular pH (pHi), it has been proposed that reduced intracellular accumulation of drugs in MDR is due to changes in the pH difference across the plasma membrane. Elevation of pHi or decrease in local extracellular pH (pHo) could reduce the intracellular accumulation of the protonated chemotherapeutic drugs and account for Pgp-mediated MDR. Alternatively, changes in pHi or pHo could increase drug efflux by other mechanisms, such as coupled transport involving H+ or OH-, or allosteric effects on Pgp or other proteins. Both mechanisms could operate independently of the charge of the substrate. The possibility of a role of pHi in drug efflux is important to test because of the clinical significance of the phenomenon of MDR of tumors. We tested this hypothesis and found that MDR can occur in cells with low, normal, or high pHi. Further, resistant cells exhibited reduced steady-state drug accumulation and increased efflux without changes in local pHo. Finally, acute changes in pHi had no appreciable effect on Pgp-mediated drug efflux. We conclude that Pgp-mediated MDR is not a consequence of changes in pHi or pHo.