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Analysis of neurotoxin and mitogen-stimulated sodium transport in human fibroblasts
Abstract:
Human fibroblasts contain two classes of ouabain-insensitive Na+ channels. One channel is activated by alkaloid neurotoxins in combination with scorpion venom, a response which is similar to that of the voltage-sensitive Na+ ionophore of nerve. The other channel is activated by fetal calf serum, several other mitogenic agents, or low extracellular Ca2+. The nervelike Na+ channel is inhibited by tetrodotoxin but not by amiloride, while the mitogen-stimulated channel is blocked by amiloride but not by tetrodotoxin. The combined effects of neurotoxins and low Ca2+ on Na+ uptake are additive, consistent with the conclusion that these Na+ pathways are different and independent. Studies of Na+ efflux show the presence of two intracellular Na+ compartments. When Na+ flux through the (Na+ + K+)-ATPase pump is inhibited by ouabain, compartment A has a high rate constant of efflux (K1 = 0.30 mn-1) and constitutes 60% of the total cellular Na+. Compartment B, with a markedly smaller efflux constant (k2 = -0.07), contains the remaining 40% of cell Na+. Stimulation of flux through the nervelike channel has a pronounced effect on the rate constants and Na+ content of both compartments (K1 = 0.56, k2 = 0.10), but activation of the mitogenic channel has a pronounced effect only on the rate of efflux from compartment A and on the size of compartment B. A cytoplasmic-nuclear model for the Na+ compartments is presented.
Insights
Human fibroblasts possess two distinct sodium (Na+) channels. One is nerve-like and sensitive to neurotoxins, while the other responds to mitogens and low calcium, impacting cellular sodium dynamics.
Area of Science:
- Cellular biology
- Ion channel physiology
- Biochemistry
Background:
- Human fibroblasts exhibit complex sodium (Na+) transport mechanisms.
- Two distinct classes of ouabain-insensitive Na+ channels have been identified.
- These channels differ in their activation and inhibition properties.
Purpose of the Study:
- To characterize the distinct Na+ channels in human fibroblasts.
- To investigate the functional differences between these channels.
- To elucidate the impact of channel activity on intracellular sodium compartments.
Main Methods:
- Utilized neurotoxins (tetrodotoxin, amiloride), scorpion venom, fetal calf serum, and varying extracellular calcium (Ca2+) concentrations.
- Measured Na+ uptake and efflux rates.
- Analyzed intracellular Na+ compartments (A and B) and their efflux rate constants.
- Proposed a cytoplasmic-nuclear model for Na+ compartments.
Main Results:
- Identified a nervelike Na+ channel inhibited by tetrodotoxin and a mitogen-stimulated channel blocked by amiloride.
- Demonstrated additive effects of neurotoxins and low Ca2+ on Na+ uptake, indicating independent pathways.
- Characterized two intracellular Na+ compartments (A and B) with distinct efflux rates.
- Showed that nervelike channel stimulation affects both compartments, while mitogenic channel activation primarily impacts compartment A and compartment B size.
Conclusions:
- Human fibroblasts possess at least two independent, ouabain-insensitive Na+ channels with different activation and pharmacological profiles.
- These channels differentially regulate intracellular Na+ compartments, influencing cellular Na+ homeostasis.
- The findings support a model of distinct Na+ pathways contributing to cellular function and signaling.