Parallel metabotropic pathways in the heart of the toad,Bufo marinus

This study examined the transduction pathways activated by epinepherine in the pacemaker region of the toad heart. Recordings of membrane potential, force, and intracellular Ca2+ concentration ([Ca2+]i) were made from arrested toad sinus venosus. Sympathetic nerve stimulation activated non-α-, non-β-adrenoceptors to evoke a membrane depolarization and a transient increase in [Ca2+]i. In contrast, the β-adrenoceptor agonist isoprenaline (10 μM) caused membrane hyperpolarization and decreased [Ca2+]i. The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.5 mM) mimicked the isoprenaline-evoked membrane hyperpolarization. Epinephrine (10–50 μM) caused an initial membrane depolarization and an increase in [Ca2+]i followed by membrane hyperpolarization and decreased [Ca2+]i. The membrane depolarizations evoked by sympathetic nerve stimulation or epinephrine were abolished either by the phospholipase C inhibitor U-73122 (20 μM) or by the blocker ofd- myo-inositol 1,4,5,-trisphosphate-induced Ca2+ release, 2-aminoethoxydiphenyl borate (2-APB, 60 μM). Neither U-73122 nor 2-APB had an affect on the membrane hyperpolarization evoked by β-adrenoceptor activation. These results suggest that in the toad sinus venosus, two distinct transduction pathways can be activated by epinephrine to cause an increase in heart rate.

Effects of sympathetic nerve stimulation on membrane potential, [Ca2+]i, and force in the toad sinus venosus

The effects of sympathetic nerve stimulation on beat rate, force, intracellular Ca2+ concentration ([Ca2+]i) measured using fura 2, and membrane potential were recorded from the spontaneously beating toad sinus venosus. Short trains of stimuli evoked an increase in the beat rate and force. During this tachycardia the amplitude of pacemaker action potentials was not changed, but there was an increase in the basal level of [Ca2+]iwith little change in peak [Ca2+]imeasured during each action potential. Depletion of intracellular Ca2+ stores with caffeine (3 mM) abolished all responses to sympathetic nerve stimulation. The effects of caffeine were fully reversible. Caffeine (3 mM), in the presence of the Ca2+-ATPase inhibitor thapsigargin (30 μM), abolished irreversibly the chronotropic and inotropic responses evoked by sympathetic nerve stimulation. Ryanodine (10 μM) attenuated, but did not abolish, these responses. These results suggest that, in the toad sinus venosus, increases in force and beat rate evoked by sympathetic nerve stimulation result from the release of Ca2+ from intracellular Ca2+ stores.