This produces constriction of the afferent arteriole. This increases the resistance of the afferent arteriole and prevents delivery of the increased mean arterial pressure beyond that point, thereby keeping RBF and GFR constant. The opposite occurs if there is a reduction in mean arteriole pressure within the autoregulatory range and a lack of stretch of the afferent arteriole. The component of tubular fluid sensed at the macula densa is not clear. The factor that is released from this region to produce constriction of the afferent arteriole is even less certain.
Some current candidates are adenosine, prostaglandins, and kinins.
To a limited extent, TGF can act to produce dilation of the afferent arteriole and an increase in GFR when there is a reduction of the sensed substance in the tubule lumen delivered to the macula densa. When this occurs, the ordinary autoregulatory process is overridden. This will lead to increased filtration before the proteins are concentrated sufficiently to produce filtration equilibrium. Increased systemic arterial pressure. This peptide is produced in granules in the left atrium.