see also calcium channel blockers, calcium

• eg. via high [K]e or depolarising electrical stimuli;
• consists of alpha1, alpha2, beta, gamma, delta subunits;
• certain snail toxins from genus Conus & Cd/Mn cations block a wide range of Ca channel subtypes;

• L:
• N: neural - important role in the spinal transmission of pain - blocked by omega conotoxin & ziconotide
• T: transient

• eg. via hormones;

• some blood vessels show increased Ca influx when stretched;

• contraction regulated by cytosolic Ca levels due to:
  • voltage-gated Ca channel via depolarisation;
  • receptor mediated hydrolysis of memb. phosphatidylinositol → IP3
    • → incr. release of Ca from sarcoplasmic reticulum;
    • → incr. Ca influx from extracellular as well;
  • receptor operated Ca channels;
• incr. cytosolic Ca → incr. Ca binding to calmodulin
  • → activ. of myosin light-chain kinase → Pn of myosin light chain
  • → incr. interaction b/n actin & myosin → contraction;
• THUS, Ca blockers relax arterial sm.m. but have little effect on most venous beds & hence do not affect cardiac preload;

• memb.depolarisation → fast Na & slow Ca channel activated
  • → cytosolic Ca incr. binds to troponin → decr. inhib. effect of troponin
  • → incr. interaction of actin/myosin;
• THUS, Ca blockers
  • → -ve inotropy;
  • → decreased automatacity & slowed conduction depend on effect on Ca channel recovery;