Step I Pancreatic secretions

[Phloston]

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Why does low flow --> increased Cl-, and high flow --> increased HCO3- ?

Aside from CFTR, there's a Cl-/HCO3- antiporter, so I'd think that a low flow rate gives more time for bicarb to enter the lumen, so low flow should = higher HCO3-. Obviously that can't be right though.

The only other thing I can think of right off the bat is that low flow gives more time for CFTR to pump Cl- into the lumen, and that HCO3- is generally just linked to PSNS function, which coincidentally coincides with increased flow (same as with saliva).

Does anyone know?

 

[Akarat]

I have in my notes that secretin stimulates high flow rates in the acinar and ductular cells of the pancreas. The acinar cells secrete the zymogens that contain digestive enzymes, while the ductular cells primarily dilute and alkalinize the pancreatic secretion.

The acinar cells have CFTR channels but they lack HCO3-/Cl- exchangers, while the ductular cells have both CFTR and HCO3-/Cl- exchangers.

I’m guessing that during periods of low flow, the ductular cells aren’t as active and so most of the pancreatic secretion is from acinar cells. Therefore, you’ll mostly have Cl- in the secretion. During periods of high flow, all of the ductular cells are actively secreting HCO3- into and picking up Cl- from the acinar secretion. Therefore, you’ll mostly have HCO3- in the secretion.

 

[Phloston]

Smart, Akarat. Thanks for that.

 

[Nabin]

Why does low flow --> increased Cl-, and high flow --> increased HCO3- ?

Aside from CFTR, there's a Cl-/HCO3- antiporter, so I'd think that a low flow rate gives more time for bicarb to enter the lumen, so low flow should = higher HCO3-. Obviously that can't be right though.

The only other thing I can think of right off the bat is that low flow gives more time for CFTR to pump Cl- into the lumen, and that HCO3- is generally just linked to PSNS function, which coincidentally coincides with increased flow (same as with saliva).

Does anyone know?

What Ganong physiology says is that HCO3 that is secreted by the smaller ductular cells is usually reabsorbed by the larger ductular cells in exchange for Chloride. When flow is high, this exchange is impaired and there is high HCO3 in the secretions and less chloride. When flow is low, this exchange has time to take place and is amplified more, causing HCO3 to get reabsorbed and more chloride to get secreted. It makes sense mechanically and physiologically too.

 

[Phloston]

What Ganong physiology says is that HCO3 that is secreted by the smaller ductular cells is usually reabsorbed by the larger ductular cells in exchange for Chloride. When flow is high, this exchange is impaired and there is high HCO3 in the secretions and less chloride. When flow is low, this exchange has time to take place and is amplified more, causing HCO3 to get reabsorbed and more chloride to get secreted. It makes sense mechanically and physiologically too.

Three years later, thanks for the reply ;-)

 

[Nabin]

Three years later, thanks for the reply ;-)

haha never too late to share something..I just found this while I was going through the topic..cheers!

 

[Transposony]

Better late than never 😏

 

[Mr.Scalpel]

Another three years later, I found a completely different and counterintuitive explanation in big Costanzo as I, too, had the same doubt when I read that in BRS.

“The relationship between flow rate and the relative concentrations of Cl− and HCO3− is explained as follows: At low (basal) rates of pancreatic secretion, the pancreatic cells secrete an isotonic solution composed mainly of Na+, Cl−, and H2O. However, when stimulated (e.g., by secretin), the centroacinar and ductal cells secrete even greater amounts of an isotonic solution with a different composition, mainly Na+, HCO3−, and H2O.”

Excerpt From: Costanzo, Linda S. “Physiology, (Costanzo Physiology).” iBooks.

It just doesn't follow the same "logical" pattern you would expect as in the salivary ducts (i.e affected by contact time).

 

[IkramKhaliq]

2020 here, lol.

Had the same confusion. Learning this concept along the following lines:

During periods of high flow of pancreatic secretions (via secretin), secretin increases the activity of the Cl-/HCO3- antiporter, thereby increasing bicarbonate content in the secretions. Conversely, during periods of low flow (when we don't really need pancreatic secretions and hence, have little secretin), the activity of this transporter would be negligible and HCO3- would not be secreted in exchange for Cl-, resulting in higher Cl- content with low flow.