According to myogenic hypothesis of renal  autoregulation,  the  afferent  arterioles contract in response to stretch induced by
Stretch on smooth muscle (afferent  arterioles) opens Ca+ channel, which causes contraction of afferent arterioles.the medullary interstitium.
REGULATION OF GFR
Regulation is done by:

Blood pressure: Autoregulation
Resistance of afferent and efferent anterioles

- Autoregulation: Constant maintenance of renal blood flow or, GFR over the span of renal perfusion pressure (pressure in renal artery minus pressure in renal vein ) from 80 to about 170mm Hg
- Mechanism of autoregulation:
i) Myogenic mechanism- contraction or relaxation of anteriolar smooth muscle in response to changes in vascular pressures; mediated by Ca++
ii) Tubule glomerular feedback (TGF) mechanism-mediated by macula densa cells (described later)
NO release
Noradrenaline release
Opening of Ca2+  channels
Adenosine release