Final answer:
A Gi protein, when activated by a ligand binding to a G-protein-linked receptor, directly opens K+ channels in the plasma membrane of heart pacemaker cells, leading to cell hyperpolarization and a slower heart rate.
Step-by-step explanation:
Gi Protein and K+ Channels in Heart Pacemaker Cells
When a ligand binds to a G-protein-linked receptor on the surface of heart pacemaker cells, it triggers a conformational change that activates the associated G-protein. Specifically, in the case of inhibitory G-proteins, or Gi proteins, this activation involves the exchange of GDP for GTP on the alpha subunit of the Gi protein. Once GTP binds, the alpha subunit dissociates from the beta and gamma subunits.
This active Gαi-GTP complex is then able to directly interact with and open K+ channels in the plasma membrane (PM). The opening of these channels allows K+ to flow out of the pacemaker cells, which leads to the hyperpolarization of the cell membrane and a decrease in the heart rate. This is part of the parasympathetic nervous system's response to slow the heart rate.
The cycle concludes with the hydrolysis of GTP to GDP, which deactivates the Gαi subunit, allowing it to re-bind to the beta and gamma subunits, making the G-protein ready for another cycle of activation.