1. 1.

    An NH4+-selective membrane for microelectrodes (NH4ISMs) was tested under ‘biological conditions’ in normal Astacus saline (NAS), two simulated intracellular salines (SIS) and the sensory neurone of the crayfish stretch receptor.

  2. 2.

    The effects of several physiological variables on intracellular NH4+ measurements were tested in vitro. Changes in the background K+ and Na+ concentrations, the ratio K+/Na+, pH, ionic strength, osmotic pressure and volume were examined.

  3. 3.

    Phenomena specific for NH4ISMs, such as a positive potential shift, an undershoot and a difference between pre- and postcalibration curves are described and discussed. We propose to consider the values of intracellular NH4+ concentration as apparent.

  4. 4.

    The detection limit of the NH4ISM is closely related to background K+ concentration. It is in the region of 0.1 mmoll−1 NH4+ in NAS (at 5.4mmol l−1 K+) and about 5mmoll−1 in SIS (at 194mmol l−1 K+).

  5. 5.

    When comparing levels of intracellular NH4+, either measured directly by NH4ISM or calculated, according to Boron and de Weer (J. gen. Physiol.67, 91–112), from simultaneously recorded pHi, we found that [NH4+]i obtained by direct measurement differed quantitatively from that of the Boron and de Weer model, but that some of the qualitative and temporal aspects of the model agreed with our results. The quantitative difference in [NH4+]i determined by the two methods cannot be attributed to temporal and/or quantitative limitations of the NH4ISM.

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