Magnetic field observations by the MESSENGER spacecraft in orbit about Mercury revealed flow of electric current from the magnetosphere to low altitudes along magnetic field lines. These field-aligned, or Birkeland, currents flow downward (blue) at dawn and upward (red) at dusk and have magnitudes 100 times smaller than at Earth. Models for the electrical conductivity of the planet suggest that the currents flow radially through the low-conductivity layers near the surface and laterally from dawn to dusk through more conductive material at depth.
The cyclic nucleotide cAMP is synthesized by adenylyl cyclase enzymes , which are downstream of heterotrimeric G-proteins ( guanine nucleotide binding proteins) and receptors. For example, when epinephrine binds to beta-adrenergic receptors in cell membranes, G-protein activation stimulates cAMP synthesis by adenylyl cyclase. The newly synthesized cAMP is then able to act as a second messenger, rapidly propagating the epinephrine signal to the appropriate molecules in the cell. This stimulatory signaling pathway leads to the production of effects such as increasing rate and force of contraction of the heart that are characteristic of epinephrine. Caffeine also enhances the action of cAMP by inhibiting the enzyme phosphodiesterase, which degrades cAMP; the enhancement of cAMP activity contributes to the general stimulatory action of caffeine. As a gas, nitric oxide (NO) is distinct among second messengers in being able to diffuse across cell membranes, which allows signal information to cross into neighbouring cells.