Regulation of pH in rat brain synaptosomes. I. Role of sodium, bicarbonate, and potassium.

1. We investigated the regulation of intracellular pH (pHi) in rat brain isolated nerve terminals (synaptosomes), using fluorescence pH indicators and time-resolved fluorescence spectroscopy. 2. The resting pHi was not significantly affected by the presence or absence of HCO3-. Removal of external Na+, in the absence or presence of HCO3- caused a rapid acidification of pHi. The recovery from acid loads was primarily due to the activity of the Na+/H+ exchanger, confirming the relevance of this transport system in synaptosomes. 3. Our data revealed that in synaptosomes the activity of the Na+/H+ exchanger was not regulated by either protein kinase C or kinase A. In contrast, Ca2+ played an important role in the regulation of Na+/H+ exchanger. This was supported by the observation that 4Br-A23187 induced a Na(+)-dependent alkalinization of the resting pHi and greatly enhanced the initial rate and the degree of the recovery from acid loads. 4. In most eukaryotic cells, HCO3(-)-based transport mechanisms play an important role in pHi regulation. In synaptosomes, however, HCO3- transport is not significantly involved in pHi regulation, because the presence or absence of HCO3- does not affect resting pHi nor the rate of pHi recovery to acid loads. Further studies to address the role of Cl- and HCO3- in pHi regulation in synaptosomes are discussed in the companion paper. 5. Increasing the concentration of Ko+ also resulted in a rise of steady-state pHi by a processes that is Ca2+ and HCO3- independent. This alkalinization could be due to either K+/H+ exchanger activity, K(+)-induced depolarization, reduction of delta microH+, or a direct reduction of delta microK+. Calculated H+ driving forces suggest that the reduction in the inwardly directed H+ leak is sufficient to explain this K(+)-induced alkalinization because it changes the delta microH+ by virtue of setting the membrane potential difference (Em) to the K+ equilibrium potential (EK+).