Preference of short isoprene-repeat terpenes for Cx43 gap junctions compared with cardiac Na+ and Ca2+ channels: application for cardiac ephaptic transmission research

Essential oils (EOs) from plants exhibit various biological activities. The major ingredients of EOs are terpenes composed of isoprene subunits. In our recent studies, we have determined several terpenes as new inhibitors of intercellular communication through gap junctions (GJ). Moreover, we showed that the potency of the monoterpene α-pinene depended on connexin 43 (Cx43) phosphorylation. In the current work, using molecular modeling, we selected terpenes composed of different number of isoprene subunits and having different number of cycles. Using patch-clamp techniques, we examined their potency to inhibit Cx43 GJ conductance. We found that the selected terpenes inhibited the Cx43 GJ conductance with IC₅₀ies ranging from 2.6 to 38 µM; the sesquiterpenes but not monoterpenes exhibited extremely long delay until the onset of GJ conductance recovery after their washout; the potency of sesquiterpenes to inhibit the membranous Na⁺ and L-type Ca²⁺ currents in human atrial myocytes was more than 10-fold lower. These properties of sesquiterpenes raised the idea of using them for verifying the controversial ephaptic excitation that we carried-out by microelectrode recording and optical mapping in Langendorff-perfused rabbit's heart. High doses of farnesene that were supposed to completely block GJ intercellular communication did not block the spread of electrical excitation in the ventricles and only produced mild changes in action potential parameters comparable to farnesene impact on membranous Na⁺ and Ca²⁺ currents. Our findings suggest that ephaptic transmission may play a major role in cardiac excitation.