EFFECT OF CHANGING TEMPERATURE ON THE IONIC PERMEATION THROUGH THE CYCLIC GMP-GATED CHANNEL FROM VERTEBRATE PHOTORECEPTORS

Native cGMP-gated channels were studied in rod outer segments of the l awal tiger salamander Ambystoma tigrinum. The a subunit of the cGMP-ga ted channel from bovine rods, here referred to as the wild type (w.t.) , and mutant channels were heterologously expressed in Xenopus laevis oocytes. These channels were studied in excised membrane patches in th e inside-out configuration and were activated by the addition of 100 o r 500 mu M cGMP. The effect of temperature on the ionic permeation was studied. The macroscopic current flowing through the native channel a t +100 mV had an activation energy of 35.8, 30, 31.8, 34.5, 41.3, and 22.4 kJ mol(-1) in the presence of Li+, Na+, K+, Rb+, Cs+, and NH4+ re spectively. The macroscopic current flowing through the w.t. channel a t +100 mV had an activation energy of 45.2, 38.2, 37.5, 47.3, 49.4, an d 38.9 kJ mol(-1) in the presence of Li+, Na+, K+, Rb+, Cs+, and NH4respectively. The activation energy of the macroscopic current flowing through the native and w.t. channels did not vary significantly when the ionic concentration of the permeant ion was changed between 2.5 an d 110 mM. The activation energy of the single-channel current of the w .t. channel at +100 mV was 40.4 and 33 kJ mol(-1) for Na+ and NH4+, re spectively, The reversal potential of biionic solutions changed signif icantly with temperature. These results can be used to obtain an estim ate of the enthalpic and entropic contributions to the barrier of the Gibbs free energy experienced by an ion during its permeation through the open channel. These estimates indicate that the ionic permeation a nd selectivity of the cGMP-gated channel are controlled both by enthal pic and entropic factors and that the selectivity of the native channe l for Li+ over Nai is primarily caused by entropic effects.