Citrulline: the pump is a vessel, not a fibre

L-citrulline is an amino acid the kidney turns into L-arginine, and arginine is the raw material nitric oxide synthase uses to make nitric oxide. Swallow arginine itself and the gut and the liver burn a lot of it with arginase before it ever reaches a vessel. Citrulline skips that first pass. The kidney runs argininosuccinate synthase and argininosuccinate lyase and hands arginine back to the blood. Endothelial nitric oxide synthase, eNOS, then splits arginine into nitric oxide and citrulline again, using oxygen and the cofactor tetrahydrobiopterin. The nitric oxide diffuses into the smooth muscle of the vessel wall, wakes soluble guanylate cyclase, and that enzyme turns GTP into cyclic GMP. Protein kinase G phosphorylates the machinery that lets the muscle of the wall relax. The vessel widens. More blood in the working muscle is the pump.

In short. Citrulline becomes arginine, arginine becomes nitric oxide, and the vessel relaxes. That extra blood is the pump. Arginine swallowed on its own mostly gets destroyed before it arrives.

The pump is fluid and substrate, not a new sarcomere. A wider vessel delivers more oxygen and more amino acids while the set is happening, and it leaves the muscle looking fuller for a while afterwards because plasma has shifted into the tissue. Myosin is added later, by tension and by food, on the fibre pages. A meta-analysis of citrulline shows a small help with repeated hard work and with how hard a set feels, and a much thinner story for a one-rep max or for measured growth. Citrulline malate is the amino acid stuck to malic acid, which is a Krebs-cycle intermediate. The malate half has been asked to carry a fatigue story it has not earned in humans. Most of what moves is still the nitric oxide half, and only when the dose of citrulline is large enough to raise plasma arginine. A scoop that tingles because it also contains beta-alanine is a different receptor, MrgprD on the skin, and it is not this pathway.

In short. Fuller is not bigger. The extra blood can make the work feel easier. It does not lay down new filaments. The tingle in a pre-workout is usually beta-alanine, a different mechanism.

Cyclic GMP does not last. Phosphodiesterase 5, PDE5, cuts it back to GMP and the vessel tightens again. Tadalafil and sildenafil occupy PDE5 and stop that cut, so whatever nitric oxide the endothelium already made hangs around longer. They do not make more nitric oxide. Citrulline does not block PDE5. One supplies the substrate. The other protects the messenger. Take both and you have stacked two steps of the same cascade, which is why the pump and the blood pressure can move further than either alone, and why a person already on a nitrate for chest pain is told to stay away from a PDE5 inhibitor. The vessel can open until the pressure falls. Headache, a blocked nose, a flushed face, and a drop when you stand up are the same enzyme in other vascular beds. The penis uses this pathway too. That is why the drugs exist. A gym pump is a skeletal-muscle side entrance to a medicine written for a different organ.

In short. Tadalafil stops the enzyme that switches the signal off. Citrulline helps make the signal. They are not the same drug. Together they can drop the blood pressure, which is the point of the warning on a nitrate.

None of this occupies the androgen receptor, none of it raises LH, and none of it keeps a beta-2 agonist off the heart. A pump that looks like growth on a photograph is still water. Read how a fibre grows for the part that adds myosin. This page is only the gas that opens the vessel, and the enzyme that decides how long it stays open.

In short. The pump is not testosterone and it is not new muscle. It is a vessel that stayed open.

eNOS, which is NOS3, sits in the caveolae of the endothelial cell tied to caveolin-1, and in that state it is quiet. A set supplies two things the enzyme was waiting for. Shear from the blood already moving phosphorylates it, and Akt can do that at serine 1177. Calcium-calmodulin pulls it off caveolin. Only then does it run. The reaction is arginine plus oxygen plus electrons from NADPH, with tetrahydrobiopterin, BH4, holding the enzyme together so the product is nitric oxide rather than superoxide. If BH4 is short, the same enzyme uncouples and makes the radical that destroys the nitric oxide you just gained. That is why a damaged endothelium does not start pumping just because the arginine number in the blood went up. The set was already a nitric oxide signal. Citrulline raises the substrate. It does not replace the shear.

In short. The enzyme that makes nitric oxide is switched on by the work itself. Citrulline only supplies the raw material. If the enzyme is broken, more raw material becomes a damaging radical instead of a wider vessel.

There is a puzzle called the arginine paradox. The enzyme's appetite for arginine is so easy to satisfy that blood levels ought to be enough already, and yet raising arginine still makes more nitric oxide in living vessels. Two local thieves explain it. Arginase sits in the same cell and eats arginine. Asymmetric dimethylarginine, ADMA, is made when proteins are broken down and it competes for the same pocket on eNOS. Citrulline, by lifting arginine after the liver has been skipped, improves the ratio of substrate to those thieves. It does not delete them. The citrulline that eNOS spits out can be turned back into arginine again, which is why the cycle has the amino acid's name on both ends.

In short. The enzyme should already have enough arginine, and yet extra still helps, because arginase and a rival molecule called ADMA steal from it. Citrulline shifts that argument. It does not end it.

Nitric oxide does not bind a receptor on the skeletal fibre and tell it to grow. It binds the haem of soluble guanylate cyclase in the smooth muscle of the vessel wall. Cyclic GMP rises. Protein kinase G phosphorylates myosin light chain phosphatase, so the wall's own actin and myosin let go, and it opens potassium channels that drop the calcium the wall needs in order to stay tight. The vessel widens. Plasma moves into the tissue. That is the mirror. The myosin in the fibre you trained is a different protein, in a different cell, listening to tension and to leucine. Confusing the two myosins is how a pump photograph becomes a growth claim. PDE5 is what cuts cyclic GMP back down. Sildenafil and tadalafil both occupy it. Tadalafil lets go more slowly, which is why one tablet is still in the chair the next day and sildenafil is largely gone. Slower off-rate is not a stronger nitric oxide synthase.

In short. The muscle that relaxes is the muscle in the vessel wall, not the fibre you trained. Tadalafil stays on the off-switch longer than sildenafil. Neither one tells the fibre to lay down filaments.