There is a population of calcium channels throughout the membrane. In this map a single channel is represented in its closed (touching paired ovals)and open (separated oval pair) state. The channel opens (dark arrow) when the membrane potential reaches +20 mV. However, the population of channels, as a whole, responds slowly. Because of this they are often called slow calcium channels. The actual number of channels that open depends on how many have bound cyclic adenosine monophosphate (cAMP) as shown in the illustration of the open channel. These channels remain open several tenths of a second before closing as indicated by the thick arrow labeled 'time'.
Adenylcyclase is a membrane-bound enzyme shown as a circle labeled ACase. When activated, it converts cytoplasmic adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP) as indicated by the reaction arrows. Cyclic AMP is rapidly decomposed into its noncyclic form (AMP) by the enzyme phosphodiesterase (PDase circle). The half-life of cAMP is brief and adenylcyclase (ACase) must be continually re-activated to maintain cAMP. While present, free cAMP equilibrates with those bound to calcium channels (double headed dashed arrow). Calcium channels with bound cAMP are more likely to open when stimulated.
The sarcoplasmic reticulum (irregular object inside cell) stores calcium ions by the action of ATP-dependent calcium pumps (twisted arrow in ATP circle). There is also a population of chemically-gated calcium channels in the membrane of this organelle. (They are shown in the open state as a pair of ovals with a block arrow between them.) These channels are in the 'closed' state when the free calcium ion level in the cytoplasm is low. Low levels are maintained by the constant activity of a the calcium pumps in the sarcoplasmic reticulum membrane as well as a large population of ATP-dependent calcium pumps in the cell membrane (ATP circle on cell membrane , not shown here).
Calcium influx (block arrow) through the cells' voltage-gated calcium channel (separated oval pair) is the initial source of cytoplasmic calcium. Some of these ions bind to calcium channels in the membrane of the sarcoplasmic reticulum and cause them to open; this is shown at the tip of the block arrow . The resulting efflux of calcium ions from the reticulum and into the cytoplasm (block arrow) accounts for ~80% of the final cytoplasmic level of these ions.
Last Updated:7/16/2005