Final answer:
Histamine signal transduction through the H1 receptor involves the cleavage of PIP₂ by phospholipase C, forming DAG and IP3. IP3 binds to ER ligand-gated ion channels, releasing calcium ions into the cytosol, which then activate proteins to induce a cellular response.
Step-by-step explanation:
Histamine Signal Transduction from the H1 Receptor
The process of histamine signal transduction through the H1 receptor involves several steps. Initially, the histamine binds to the H1 receptor, activating the associated G protein. This activated G protein then stimulates the enzyme phospholipase C (PLC). PLC cleaves a specific phospholipid in the plasma membrane called PIP₂, leading to the formation of two second messengers: diacylglycerol (DAG) and inositol triphosphate (IP3).
DAG remains in the plasma membrane and plays a role in activating protein kinase C (PKC), which phosphorylates proteins and triggers a cellular response. Meanwhile, IP3 diffuses through the cytoplasm and binds to ligand-gated ion channels in the membrane of the endoplasmic reticulum (ER). This binding causes the channels to open, allowing calcium ions (Ca²⁺) to flow out of the ER, raising the cytosolic calcium ion concentration. These calcium ions then either directly mediate cellular effects or bind to proteins like calmodulin to further propagate the signal within the cell.
Therefore, in the context of the options provided to complete the flowchart of histamine signal transduction, the correct sequence after the PLC cleaves PIP₂ would be:
- Enzyme cleaves PIP₂, forming DAG and IP3
- IP3 binds to a ligand-gated ion channel in the ER membrane
- Calcium ion concentration increases in the cytosol
- Calcium ions activate a protein, leading to a cellular response