These experiments examine changes in the agonist-induced conductance
that occur when the agonist-receptor complex is perturbed. Voltage-clamped
Electrophorus electroplaques are exposed to the photoisomerizable agonist
trans-Bis-Q A 1-µs laser flash photoisomerizes some trans-Bis-Q molecules
bound to receptors; because the cis configuration is not an agonist, receptor
channels close within a few hundred microseconds. This effect is called phase 1.
We compare (a) the fraction of channels that close during phase 1 with (b) the
fraction of trans-Bis-Q molecules that undergo trans → cis photoisomerization.
Parameter a is measured as the fractional diminution in voltage-clamp currents
during phase 1. Parameter b is measured by changes in the optical spectra of
Bis-Q solutions caused by flashes . At low flash intensities, a is twice b, which
shows that the channel can be closed by photoisomerizing either of two bound
agonist molecules. Conventional dose-response studies with trans-Bis-Q also give
a Hill coefficient of two. As a partial control for changes in the photochemistry
caused by binding of Bis-Q to receptors, spectral measurements are performed
on the photoisomerizable agonist QBr, covalently bound to solubilized acetylcholine
receptors from Torpedo. The bound and free agonist molecules have the
same photoisomerization properties. These results verify the concept that the
open state of the acetylcholine receptor channel is much more likely to be
associated with the presence of two bound agonist molecules than with a single
such molecule
