Arrange the following physiological events in the sequence that occurs on exposure to hypoxia: A. Reduced conductance of hypoxia-sensitive K+ channels B. Ca2+ influx C. Decreased K+ efflux D. Exocytosis
First, I need to recall what happens in the body during hypoxia. Hypoxia refers to low oxygen levels. In tissues like the lungs, especially in the pulmonary arteries, hypoxia causes vasoconstriction. But the question is about the sequence of events in the cells.
Starting with the potassium channels. I remember that in pulmonary artery smooth muscle cells, hypoxia leads to the closure of ATP-sensitive K+ channels. Wait, the question mentions hypoxia-sensitive K+ channels. So when these channels close, potassium efflux decreases. That would be option C: decreased K+ efflux. But the options here are A: Reduced conductance of hypoxia-sensitive K+ channels. So maybe the first step is the reduced conductance (A) leading to decreased efflux (C).
Then, with decreased K+ efflux, the cell membrane depolarizes. Depolarization would open voltage-gated calcium channels, leading to calcium influx (B). The calcium influx then causes contraction of the smooth muscle cells, leading to vasoconstriction. Exocytosis (D) might refer to the release of vasoactive substances, but I'm not sure if that's part of the immediate sequence.
So the sequence would be A → C → B → D. Let me check again. Hypoxia causes closure of K+ channels (A), leading to decreased efflux (C), depolarization, Ca2+ influx (B), and then exocytosis (D) as part of the vasoconstriction process. Exocytosis could be releasing substances like endothelin or other mediators, but the primary sequence is about the ionic changes leading to vasoconstriction.
Wait, the options given are A, C, B, D. So the correct order is A → C → B → D. Therefore, the correct answer would be A, C, B, D. Let me verify each step.
Hypoxia → reduced K+ channel conductance (A) → decreased K+ efflux (C) → cell depolarizes → Ca2+ influx (B) → exocytosis (D). That makes sense. Each step follows the previous one logically. The options are in the order A, C, B, D. So the answer is A, C, B, D. Therefore, the correct sequence is A → C → B → D.
**Core Concept**
Hypoxia-induced pulmonary vasoconstriction involves a sequence of ion channel and cellular events in vascular smooth muscle. Key steps include potassium channel closure, membrane depolarization, calcium influx, and subsequent vasoactive responses.
**Why the Correct Answer is Right**
Exposure to hypoxia reduces conductance of **hypoxia-sensitive K⁺ channels** (A), leading to **decreased K⁺ efflux** (C) due to channel closure. This causes membrane depolarization, opening **voltage-gated Ca²⁺ channels** and allowing **Ca²⁺ influx** (B). Elevated intracellular Ca²⁺ triggers **exocytosis** (D) of vasoconstrictors like