Effect of hypercalcemia in heart cells. - USMLE Forums
USMLE Forums Logo
USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
Home
USMLE Articles
USMLE News
USMLE Polls
USMLE Books
USMLE Apps
Go Back   USMLE Forums > USMLE Step 1 Forum

USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam


Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 02-03-2016
USMLE Forums Newbie
 
Steps History: ---
Posts: 2
Threads: 1
Thanked 0 Times in 0 Posts
Reputation: 10
Help Effect of hypercalcemia in heart cells.

Hello everybody!

I've got a question which bothers me a long time...
Excess of calcium has as result symptoms such as muscle weakness which happen due to the anesthetic effect of calcium in Na+ channels.
Why does not the same happen with the fast Na+ channels of the heart muscle? The heart muscle shows no signs or problems of excitation in excess of calcium only a shortened Q-T.
Please, does anyone have an explanation for this?
Thanks in advance!
Reply With Quote Quick reply to this message



  #2  
Old 02-03-2016
gokulramani's Avatar
USMLE Forums Addict
 
Steps History: 1 + CS
Posts: 114
Threads: 13
Thanked 35 Times in 25 Posts
Reputation: 45
Default

Quote:
Originally Posted by geokar View Post
Hello everybody!

I've got a question which bothers me a long time...
Excess of calcium has as result symptoms such as muscle weakness which happen due to the anesthetic effect of calcium in Na+ channels.
Why does not the same happen with the fast Na+ channels of the heart muscle? The heart muscle shows no signs or problems of excitation in excess of calcium only a shortened Q-T.
Please, does anyone have an explanation for this?
Thanks in advance!
Its actually a bit more complex.

In skeletal muscles, the excitability is decreased, yes. But its not because of calcium directly causing issues on the muscle. Its related to the neurons becoming less excitable(another complicated mechanism).
Skeletal muscle contraction is independent of extracellular calcium, the calcium involved in the excitation-contraction coupling of skeletal muscles, directly comes from the Sarcoplasmic reticulum, in response to threshold-depolarizing wave, from the end-plate of the neuromuscular junction.

In cardiac tissues, neurons play no role in initiating the excitation contraction coupling in the ventricles [only the nodal tissue, sympathetics and parasymphetics control heart rate(cronotropy), contractity(inotropy), relaxation(dromotropy)].
Extracellular calcium goes in, activates the L-type calcium channels, and induces a Calcium-induced calcium release.
In this particular scenario in your question, there is excess calcium outside. This calcium is going to force its way into the cardiac tissue (think of this as, air flowing from a region of high pressure(extracellular space) to a region of low pressure (cardiac myosite).

So in this scenario, since Ca goes in fast, phase 2(calcium influx) is accelerated, so we see the ECG changes of QT shortening.

Hope this helped.
Reply With Quote Quick reply to this message
The above post was thanked by:
datson (02-06-2016), geokar (02-05-2016)
  #3  
Old 02-05-2016
USMLE Forums Newbie
 
Steps History: ---
Posts: 2
Threads: 1
Thanked 0 Times in 0 Posts
Reputation: 10
Default

gokulramani thank you very much for taking the time and answering.

I've got now some hints but actually my question was:

Will there be any conduction problems in the myocardium during hypercalcemia?
The are two main theories about Ca++ and it's interaction with Na+ channels:
1) Ca++ blocks Na+ channels by getting stuck in the channel's pore
2) Ca++ alters the resting membrane potential (it makes it more negative) around the area of the Na+ channel. So it keeps the channel in a deactivated state and is less likely that the channel will open. (a stronger current is needed to open the channel).
That's also why Ca++ is known for it's membrane stabilizing activity, because is stabilizes the Na+ channels in the deactivated state. That is also the reason that CaGluc is given in cases of hyperkalemia.

Fast Na+ channels are essential for the cardiac myocytes because when they open the membrane depolarises and the the voltage gated calcium channels open as a respond to the depolarization (voltage gated Ca++ channels open only in depolarised membrane state).

So my question is: If we have a scenario of excess of calcium extracellularly won't there be any conduction problems? Won't there be any problems with the opening of fast Na+ channels in the myocardium and therefore problems with the excitation of the heart muscle?
Reply With Quote Quick reply to this message



Reply

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

Favorite USMLE Books
What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
Where you live. Leave blank if you don't want to tell.

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
Effect of Class I (a, b, + c) drugs on Pacemaker cells? ReggieMiller USMLE Step 1 Forum 0 06-27-2013 04:06 AM
Effect of ACE inhibitors and spironolactone on the heart heartbeat USMLE Step 2 CK Forum 3 08-25-2012 04:48 PM
Chloroquine effect on pancreatic beta cells khushboo USMLE Step 1 Forum 7 11-21-2011 02:04 PM
beta blocker effect on a denervated heart! INCOGNITO USMLE Step 1 Forum 6 10-25-2011 03:03 PM
The effect of heart rate on systolic and diastolic pressures? eesfee USMLE Step 1 Forum 2 05-25-2011 12:24 PM

RSS Feed
Find Us on Facebook
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

USMLE® & other trade marks belong to their respective owners, read full disclaimer
USMLE Forums created under Creative Commons 3.0 License. (2009-2014)