Osteomalacia lab findings question

[tesladr]

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I'm having trouble wrapping my head around the lab findings in osteomalacia.

Why would you have increased alkaline phosphatase levels (indicating osteoblast activity) in osteomalacia, when you have increased PTH levels in response to the low calcium levels? Couldn't the increased PTH cause the osteoblasts to activate the osteoclasts instead, therefore leading to more clast activity and DECREASED ALP? A mechanism besides 'compensatory osteoblast activity' would help because I'm becoming OCD about this.

 

[Jabbed]

I'm having trouble wrapping my head around the lab findings in osteomalacia.

Why would you have increased alkaline phosphatase levels (indicating osteoblast activity) in osteomalacia, when you have increased PTH levels in response to the low calcium levels? Couldn't the increased PTH cause the osteoblasts to activate the osteoclasts instead, therefore leading to more clast activity and DECREASED ALP? A mechanism besides 'compensatory osteoblast activity' would help because I'm becoming OCD about this.

Osteoclast activation is dependent upon osteoblast activity (it expresses RANKL). Any increased activity in either will result in an elevated alk phos.

↑ PTH → ↑ osteoblast expression of RANKL → ↑ osteoclast activity

 

[tesladr]

Osteoclast activation is dependent upon osteoblast activity (it expresses RANKL). Any increased activity in either will result in an elevated alk phos.

↑ PTH → ↑ osteoblast expression of RANKL → ↑ osteoclast activity

Thanks for your response. I didn't realize ALP would be raised even during clast activation...however, what would be the point of increased ALP levels (creating an alkaline environment) when you need an acidic environment for the osteoclasts to do their job?

Edit: I know osteoclasts uses carbonic anhydrase to create the acidic environment, but still I ask the question 🙂

 

[Jabbed]

Thanks for your response. I didn't realize ALP would be raised even during clast activation...however, what would be the point of increased ALP levels (creating an alkaline environment) when you need an acidic environment for the osteoclasts to do their job?

I think that you're confusing form and function. Increased alkaline phosphatase activity is simply a marker of osteoblast activity-- while the purpose is to establish an alkaline environment for osteoid deposition, it will always be elevated when there is increased osteoblast activity regardless of whether or not there is net osteoid deposition. The thought paradigm should be ↑alk phos = ↑ osteoblast activity =/= ↑ net osteoid deposition