Ionically induced volume changes of the smooth muscle of the guinea-pig taenia coli

Sodium/sodium exchange in the smooth muscle of the guinea-pig taenia coli

External Na has been shown to initiate a loss of 24 Na from high-Na smooth muscle of the guinea-pig taenia coli. This is an ouabain-insensitive effect, and there appears to be a 1:1 exchange of Na ions, suggesting a classical Na/Na exchange mechanism. 2. The Na/Na exchange has many properties in common with a similar exchange studied in high-Na beef erythrocytes by Motais (1973), and Motais & Sola (1973). It is very temperature-sensitive, it is partially inhibited by the sulphydryl reagents studied and it has a fairly low external affinity for Na. 3. The affinity of the external site for alkali metal cations is Na greater than Li greater than K greater than Rb greater than Cs. 4. It has proved impossible to estimate the affinity of the intracellular sites for Na. The curve relating intracellular Na content with the stimulated efflux reaches a maximum and then declines slightly. 5. Another unexpected finding was that after the rate of loss of Na has been reduced in a Na-free medium, reintroducing Na causes an overshoot in the rate, it increases to a value beyond the original one, and then slowly declines to it. 6. Unlike the classical Na/Na mechanism, the process is reduced, but not abolished by metabolic inhibition that depletes the tissue of ATP. 7. The results are interpreted to suggest that the Na/Na exchange is occurring from a cellular compartment of limited volume, which is itself exchanging with the main cell compartment. It is suggested that this small compartment is the sarcoplasmic reticulum, and the effect of metabolic inhibition is to interfere in some way with the relationship between this compartment and the cell membrane.

Potassium accumulation in smooth muscle and associated ultrastructural changes

1. The water content, extracellular ((60)CoEDTA) space, ionic composition and ultrastructure of several mammalian smooth muscles were studied after incubation in solutions of varying ionic compositions and osmolarities.2. Substitution of KCl for NaCl resulted in an increase in cell water, K and Cl, accompanied by little change in total wet weight. This was due to a reduction in the extracellular space.3. Changes in extracellular osmolarity produced a wider range of cell volumes in high KCl solutions than in Krebs. The addition of 29-58 mM sucrose to high KCl prevented the swelling.4. Electron microscopy of smooth muscle swollen in high KCl solution revealed light (less electron opaque than normal) fibres of increased diameter, reduction in extracellular space, and nuclear swelling. The normal thick filament lattice was destroyed in swollen, osmium-fixed smooth muscles.5. The ultrastructural changes ascribed to swelling were absent in smooth muscles, (a) depolarized in high K(2)SO(4) solutions, (b) in high KCl solutions with 29-58 mM sucrose, and (c) returned to normal Krebs solution for recovery from swelling.6. Smooth muscles incubated in high KCl (swollen) and high K(2)SO(4) (unswollen) exhibited similar contractile responses, suggesting the filament lattice was intact until fixation, and that the contractile mechanism can operate over a relatively wide range of actin to myosin separations.7. Shrinkage of smooth muscles in high KCl solutions made hypertonic with the addition of 10% sucrose was accompanied by an aggregation of the thick filaments.8. The cell water of fixed taenia coli was reduced (a) by incubation in hypertonic solution followed by fixation in normal glutaraldehyde, or (b) by fixation of normal tissue in hypertonic glutaraldehyde. Osmotic responses during aldehyde fixation may be a source of artifact in the visualization of the normal filament lattice.