Connexin 32 gap junctions enhance stimulation of glucose output by glucagon and noradrenaline in mouse liver

Gap junctions connect neighboring cells via intercellular channels composed of connexins (Cx). Connexin 32 (Cx32) is the main connexin in hepatocytes. Gap junctions propagate a signal from periportal to perivenous hepatocytes generated by electrical stimulation of sympathetic liver nerves. Therefore, it was the aim of this study to examine the involvement of hepatocellular gap junctions in hormonal regulation. In perfused livers from wild-type mice and Cx32-deficient mice, the stimulation of glucose release by varying noradrenaline and glucagon concentrations was investigated. At saturating hormone concentrations, glucose release was the same in wild-type and Cx32-deficient livers. However, glucose output was significantly smaller in Cx32-deficient than wild-type livers at half-maximally effective hormone concentrations. Because the two hormones circulate at less than half-saturating concentrations and because they are degraded during passage of blood through the liver, they lose efficiency from the periportal to the perivenous zone. In wild-type livers, this decrease in efficiency can be partially compensated by intercellular signal propagation through gap junctions, resulting in higher hormone actions than in Cx32-deficient livers. It is concluded that gap junctions are not only involved in intercellular propagation of nervous, but also of hormonal signals from periportal to perivenous hepatocytes.