In act i scence iv how does macbeth react to duncans announcement that malcolm will be next in line to the throne?

When Duncan announces his decision to make his son Malcolm the heir to the throne of Scotland (something that would not have happened automatically, since his position was elected and not inherited). Duncan then states that he plans to visit Macbeth at his home in Inverness. Macbeth leaves to prepare his home for the royal visit, pondering the stumbling block of Malcolm that now hinders his ascension to the throne and what he believed was the witches' prophecy.