Sodium Dependent Mechanisms Regulating Membrane Potassium (86Rb+) Permeability in Mammalian Exocrine Glands In Vitro

Pediani, John D (1994) Sodium Dependent Mechanisms Regulating Membrane Potassium (86Rb+) Permeability in Mammalian Exocrine Glands In Vitro. PhD thesis, University of Glasgow.

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Fragments of rat submandibular gland were pre-loaded with 86Rb+, a radioactive isotopic marker of cellular K+ permeability, and rate constants for 86Rb+-efflux were determined during superfusion with unlabelled experimental physiological saline solutions which were buffered with either CO2/HCO3- or HEPES/NaOH. Initial 86Rb+-efflux experiments demonstrated that in the presence of external Na+ ([Na+]0), acetylcholine (ACh) could evoke a rapid increase in membrane K+ permeability. This increase could be resolved into two separate components, a Ca2+-independent transient phase which was attributed to the mobilisation of Ca2+ from internal stores and a Ca2+-dependent sustained phase which was evoked when extracellular Ca2+ ([Ca2+]0) was transported into the rat submandibular acini. This biphasic response to ACh was slightly impaired under HCO3-free conditions. It has been suggested that receptor-regulated Ca2+-influx in exocrine organs occurs via a Na+-dependent mechanism (Gallacher & Morris,1987), so the effects of removing external Na+ ([Na+]0) upon this biphasic increase in membrane K+ permeability was investigated. The impermeant cation N-Methyl-D-Glucammonium (NMDG+) or the permeant cation lithium (Li+) were used as Na+ substituents. It was found that the sustained response to ACh was significantly inhibited. These findings therefore supported Gallacher & Morris's (1987) hypothesis that Ca2+-inflow, which supports the sustained increase in the K+ efflux rate from rodent salivary acini, occurs via a Na+-dependent process (Gallacher & Morris, 1987). Although the data from the Na+-free experiments supported Gallacher and Morris's hypothesis, the data from these experiments also suggested that NMDG+ may exert an inhibitory effect on the transient response (Ca2+ mobilisation). This slight inhibition could be a direct effect of NMDG+ or could be due to the initially very low Ca2+ (0.02 mumol 1-1) composition of these HEPES-buffered, NMDG+-solutions since the salivary fragments would have been exposed to an outwardly directed Ca2+ gradient. The effects of NMDG+ upon membrane K+ permeability in the rat submandibular gland were therefore re-examined using HCO3-buffered, NMDG+-solutions in which the [Ca2+]0 was never lower than 0.2 mumol 1-1. The results from these experiments demonstrated that both components of the response to ACh were significantly inhibited in the presence of NMDG+. These latter findings, therefore do not support the view of Gallacher & Morris (1987) that only Ca2+-inflow into rodent submandibular acini is inhibited in the absence of [Na+]o. Furthermore, these findings contrast with analogous NMDG+-experiments undertaken in the human sweat gland where only Ca2+ mobilisation appears to he Na+ dependent (Wilson, Bovell, Elder, Jenkinson & Pediani, 1990). The physiological basis for this dependence upon [Na+]0 is not yet known. However, one hypothesis is that if proton (H+) extrusion via the Na+-H+ exchanger is blocked in the presence of NMDG+, then the resultant fall in intracellular pH (pHi) could inhibit the mobilisation of Ca2+ from internal and external pools (Grinstein & Goetz, 1985; Siffert & Akkerman, 1987; Gallacher & Morris, 1987). It is thus possible that the inhibitory effects evoked by NMDG+ in the rat submandibular gland and human sweat gland may also be due to inhibition of this transport system. This possibility was therefore investigated by examining the degree to which amiloride, a potent inhibitor of Na+-H+ exchange, could impair the cholinergic regulation of membrane K+ permeability in the rat submandibular gland and the human sweat gland. Amiloride (1 mmol 1-1) did not affect membrane K+ permeability in the rat submandibular gland, but this compound did, however, impair the regulation of membrane K+ permeability in the human sweat gland. These data suggested that the ACh-evoked K+ permeability coupling process in the human sweat gland is more sensitive to a fall in pHi. Another thesis put forward to explain this Na+-dependency is that the transport of [Ca2+]0 into rodent submandibular acini occurs via 'reversed' Na+-Ca2+ exchange i.e. [Na+]i exchanged for [Ca2+], (Gallacher & Morris, 1987). I therefore used 45Ca2+ to monitor the transport of Ca2+ in the rat submandibular gland. The data from these experiments demonstrated that the basal 45Ca2+ efflux rate was unaffected when either Na+ or Ca2+ was removed from the superfusing control solution. These results therefore suggest that Na+-Ca2+ exchange does not play an important role in maintaining low internal Ca2+ in this tissue and it is therefore very unlikely that Ca2+-influx occurs via 'reversed' Na+-Ca2+ exchange. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: S M Wilson
Keywords: Physiology
Date of Award: 1994
Depositing User: Enlighten Team
Unique ID: glathesis:1994-74787
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 27 Sep 2019 16:19
Last Modified: 27 Sep 2019 16:19

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