Muscarinic receptors and responses in intact embryonic chick atrial and ventricular heart cells

We have studied muscarinic acetylcholine (ACh) receptors in intact atrial and ventricular heart cells dissociated from 8-day chick embryos and maintained in sparse cell cultures. Two specific antagonists, [³H]quinuclidinyl benzilate (QNB) and [³H]N-methyl scopolamine (NMS), bind to surface sites with affinity ($\text{K}{}_{\mathrm{<mtext>d</mtext>}}\text{s}\text{? 40}\text{and}\text{400}\text{p}\text{M}$, respectively). The concentration of [³H]QNB sites in ventricular cell cultures (460 fmole/mg protein) was comparable to the concentration of sites in atrial cultures (420 fmole/mg protein). The same result was obtained with [³H]NMS. Autoradiography following incubation in saturating concentrations of [³H]QNB shows that nearly all of the atrial and ventricular myocytes were labeled and that the distribution of grains over individual cells was uniform. The mean binding site density was 109/μm² for atrial cells 117/μm² for ventricular cells. In contrast to the antagonist binding results, microelectrode recordings from individual myocytes or from small clusters of cells showed that many more atrial myocytes (89%) were sensitive to 10^?4M carbachol than were ventricular myocytes (26%). Saline extract of embryonic brain tissue added to the culture medium did not alter the number or distribution of ligand binding sites but it produced a 2.6-fold increase in the number of carbachol-sensitive ventricular cells.