This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Picking up post #30: that is the part I would want checked first.
Amylin signalling reaches satiety through a distinct receptor complex, which is the mechanistic basis for expecting an amylin analogue and an incretin agonist to add rather than overlap.
Ghrelin receptor agonism drives growth hormone release in pulses and also increases appetite, which is the effect people most reliably report and least often want.
Glucagon receptor agonism raises energy expenditure and promotes hepatic fat oxidation. In a triple agonist the incretin limbs offset the glycaemic consequence, which is why the combination is not self-defeating.
Posted with less confidence than the sentence structure implies.
I had written a reply contradicting post #32 and deleted it. Here is what survived.
Receptor desensitisation and internalisation are real phenomena in vitro and their clinical relevance to these compounds is not established. That distinction gets lost in discussions about tolerance.
The short version is the first sentence; the rest is why.
This follows post #34 rather than contradicting it.
The area postrema sits outside the blood-brain barrier and is where a great deal of the nausea signalling in this class originates. That is why the effect is central and not gastric irritation.
I would want to see it done twice before believing it once.
Bookmarking this. I will come back when I have something worth adding.
Central versus peripheral action: GLP-1 agonism works through both central nervous system effects (appetite) and peripheral effects (gastric motility, insulin). The balance is not fully characterised.
It cost nothing to check and would have cost something not to.
Cross-reactivity and selectivity: the compounds are not perfectly selective for their target receptors. Semaglutide has some activity on other receptors; tirzepatide activates both GLP-1 and GIP with different affinities. The off-target effects are part of the overall pharmacology.
This is the sort of thing that ought to be settled and apparently is not.
Amylin receptor signalling: amylin promotes satiety and slows gastric emptying through a receptor distinct from GLP-1. The hypothesis behind combination therapy is two complementary satiety mechanisms.
Glucagon receptor agonism: glucagon receptor agonism increases energy expenditure and promotes hepatic fat oxidation. The mechanism is distinct from GLP-1 and GIP agonism and the clinical consequences are still being characterised.
Everything in post #42 holds. The case it does not cover is the one I have.
GIP receptor signalling: the glucose-dependent insulinotropic peptide receptor (GIP) is involved in glucose-stimulated insulin secretion. GIP agonism is thought to contribute to tirzepatide's effect but the mechanism is not fully settled.
The confident version of this sentence would be wrong, so here is the hedged one.
Narrowing post #42, because the general version has more than one answer.
GLP-1 receptor signalling: the GLP-1 receptor is expressed on beta cells (insulin secretion), on neurons (appetite and gastric motility), and on myocardium (contractility). Different tissues respond to the same signal in different ways.
I keep a log of this specifically because memory is unreliable about it.
Long-term receptor changes: very little is known about what happens to receptor expression, signalling, and downstream effects over years of exposure to these compounds. That is exactly the knowledge gap phase 3 trials exist to fill.
This is where my knowledge stops and I would rather mark the edge than blur it.
Seconded. It reads as careful rather than confident, which is the right register.
Pharmacological class effects: all GLP-1 agonists slow gastric emptying and suppress appetite. Those are class effects, not unique to one compound. Effects that differ between compounds are usually about potency or receptor selectivity.
The arithmetic in post #45 is right; the assumption feeding it is the part to check.
GLP-1 receptor agonism produces its metabolic effects through more than one route: central satiety signalling, delayed gastric emptying, and glucose-dependent insulin secretion. Attributing everything to one of them is where most simplified accounts go wrong.
I had written a reply contradicting post #45 and deleted it. Here is what survived.
Endogenous versus pharmacological receptor engagement differ in magnitude and in duration by orders of magnitude. Arguments from "it is a natural hormone" do not survive that.
That is the version I use. It may not be the version that is correct.
Nothing in receptor biology tells you what is in the vial, which is worth remembering when a mechanistic thread starts being used to justify a sourcing decision.
That holds for the case as described. Change the assumptions and it may not.
Glucagon receptor agonism: glucagon receptor agonism increases energy expenditure and promotes hepatic fat oxidation. The mechanism is distinct from GLP-1 and GIP agonism and the clinical consequences are still being characterised.
Helpful, and short, which on this subject is harder than long.
Post #51 answers the question as asked. The question underneath it is different.
Central versus peripheral action: GLP-1 agonism works through both central nervous system effects (appetite) and peripheral effects (gastric motility, insulin). The balance is not fully characterised.
One more caveat and then I will stop qualifying: the sample selected itself.
Cross-reactivity and selectivity: the compounds are not perfectly selective for their target receptors. Semaglutide has some activity on other receptors; tirzepatide activates both GLP-1 and GIP with different affinities. The off-target effects are part of the overall pharmacology.
I would want the raw data before agreeing with my own summary of it.
Taking post #53 at face value and following it one step further.
Pharmacological class effects: all GLP-1 agonists slow gastric emptying and suppress appetite. Those are class effects, not unique to one compound. Effects that differ between compounds are usually about potency or receptor selectivity.
Worth saying I have only my own numbers here, and n is small.
Post #54 and I disagree about the size of the effect, not about the direction.
Long-term receptor changes: very little is known about what happens to receptor expression, signalling, and downstream effects over years of exposure to these compounds. That is exactly the knowledge gap phase 3 trials exist to fill.
Not a strong opinion, just a consistent one.
GLP-1 receptor agonism produces its metabolic effects through more than one route: central satiety signalling, delayed gastric emptying, and glucose-dependent insulin secretion. Attributing everything to one of them is where most simplified accounts go wrong.
One case, stated as one case.
Useful. I had the fact and not the reason, which turns out to be the important half.
In vitro potency and clinical potency are related by a long chain of assumptions. A compound more potent at the receptor is not necessarily more effective at a tolerable dose.
A weak preference rather than a position.