Tritiated Saxitoxin Binding in the Vertebrate Heart
Tanaka, Jacqueline Creeks
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Permalink
https://hdl.handle.net/2142/71414
Description
Title
Tritiated Saxitoxin Binding in the Vertebrate Heart
Author(s)
Tanaka, Jacqueline Creeks
Issue Date
1981
Department of Study
Physiology and Biophysics
Discipline
Physiology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Animal Physiology
Abstract
Labelled toxins such as ('3)H-STX which selectively bind to the sodium channel have been used to estimate the density of channels on the membrane. While many biochemical investigations of excitable tissues have been made on nerve, skeletal muscle or electric organ with ('3)H-STX, relatively little attention has been given to cardiac tissue.
('3)H-STX with a specific activity high enough to measure the binding properties of plasma membrane fractions was prepared. Frog heart, Rana pipiens, has a high affinity saturable binding site (K(,d) = 7 nM) on the plasma membrane. A linear component of binding was measured in the presence of 10('-5) M unlabelled STX and subtracted from the total binding; the saturable site density was 0.55 pmol/mg of protein. In addition to the plasma membrane site, a high affinity (K(,d) = 2 nM) STX binding site was found in the cytosolic frog heart fractions. The concentration of sites in this fraction was 1.8 pmol/mg protein. The cytosolic site represents 85-95% of the total STX binding in frog heart.
Measurements of the cytosolic fraction from Electrophorus electricus electric organ did not demonstrate an STX binding site suggesting that frog soluble site is not a common property of all excitable tissues. The plasma membrane fractions of electric organ, however, show a high affinity (K(,d) = 9 nM STX) site with a density of 1.9 pmol/mg membrane protein.
Neither chicken nor turtle heart have a measurable number of STX binding sites in the cytosolic fractions. Chicken heart plasma membrane fractions bound STX with a relatively low affinity (K(,d) (TURN)2 x 10('-6)M STX). Electrophysiological measurements of STX binding parameters on adult chicken heart were not available but the reports of TTX and STX sensitivity on rabbit and guinea pig heart are consistent with a low affinity sodium channel in these hearts. Turtle heart, Chrysemys scripta elegans, plasma membrane sites have a low affinity to STX also (K(,d) = 1.4 x 10('-6)M). Bufo marinus heart preparations have binding properties similar to those of Rana pipiens and Rana catesbeiana; a cytosolic site had a K(,d) = 1.4 x 10('-9) M and the plasma membrane fraction K(,d) was 1.7 x 10('-8) M STX.
By these measurements on various hearts three different STX binding sites are demonstrated; the first is a high affinity plasma membrane site as seen in nerve, electric organ and amphibian heart. The second is a high affinity cytosolic site seen in amphibian heart preparations also and the third is a low affinity site seen in chicken and turtle heart plasma membranes.
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